Kelly And Olsen 1993

Sedimentary Geology, 85 (1993)339-374

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Elsevier Science Publishers B.V., A m s t e r d a m

Terminal fans

a review with reference to Devonian examples S e a n B. K e l l y a a n d H e n r i k O l s e n b

a Department of Geology, Unit'ersity College Cork, Ireland h Department of Geology and Geotechnical Engineering, Danmarks Tekniske Hojskole Building 204, DK-2800 Lyngb~'. Denmark Received August 17, 1992: revised version accepted D e c e m b e r 1, 1992

ABSTRACT Kelly, S.B. and Olsen, H., 1993. Terminal f a n s - - a review with reference to Devonian examples. In: C.R. Fielding (Editor), Current Research in Fluvial Sedimentology. Sediment. Geol., 85: 339-374. Terminal fans occur where sediment-laden streams decrease in size and vanish as a result of evaporation and transmission losses. They tend to form in arid or semi-arid regions which are characterized by a moisture deficit. Distributary channel patterns are characteristic of terminal fans, and reflect both loss of stream power and spatially/temporally fluctuating discharge. In a n u m b e r of Devonian basins, terminal fan sediments form conspicuous sequences with examples from Spitsbergen, England, Ireland and Greenland. Examples of terminal fan systems from the Northeast Greenland Basin and the Munster Basin in Ireland are presented in this paper. The ancient examples are used in combination with modern distributary systems to construct a simple facies model for terminal fans and their deposits. The model includes a tripartite zonation of terminal fans into feeder, distributary and basinal zones. The feeder zone is characterized by large channel bodies associated with interchannel fines. An increase in channel body frequency may occur at the transition from the feeder zone to the distributary zone, reflecting the downstream multifurcation of channels. The distributary zone is characterized by a downstream decrease in both the scale and frequency of channel deposits, which are mainly replaced by sheetflood deposits. This is the result of the decline of both water depth and stream power downslope. Further evidence of terminal fan systems is the downstream transition from distal to basinal zone deposits of floodbasin, playa mudflat or aeolian origin, reflecting the absence of a terminal base level in the form of a lake or the sea.

Introduction Several ancient fluvial systems can be interpreted as terminal fans (Friend, 1978), and "as our understanding of 'terminal fan' systems increases it would seem there is as great a variability of facies models as is already well-perceived for meandering and braided stream systems" (Tunbridge, 1984, p. 713). The purpose of this

Correspondence to: Sean B. Kelly, G e o c h e m Group Limited, Unit 3, Commerce Centre Souter Head Road, Altens Industrial Estate, Aberdeen AB1 4LF, UK.

p a p e r is to review fluvial sand-dominated and mixed-load terminal fans, both ancient and modern, in terms of process and product. Initially, the processes characteristic of terminal fans and related systems are discussed and incorporated into a generalised environmental model. The applicability of this model is then illustrated with several examples of M i d d l e - U p per Devonian terminal fan systems from Greenland and Ireland. Finally, a general facies model is proposed which incorporates the observations from both modern and ancient systems. It is hoped that this model may serve as a framework for future studies of terminal fan systems.

0037-0738/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

340

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Terminal fans

The term terminal fan is used here to describe fluvial distributary systems in which the drainage is wholly dissipated internally via a distributary network from which no water escapes by surface flow to a take or the sea during normal conditions. There are notable differences between conventional coarse-grained alluvial fans (e.g. Heward, 1978) and the sand-dominated/mixed-load, lowgradient, terminal fan systems considered in this paper. Alluvial fans generally result from a marked drop in local base level, e.g. along mountain fronts, typically "where a heavily laden stream reaches the plain after flowing swiftly through a ravine or canyon" (Holmes, 1965). In contrast, the parent river of a terminal fan is generally mobile and not restricted to a ravine or canyon. Terminal fans develop primarily as a response to high evaporation/low precipitation rates (i.e. moisture deficit) and high infiltration rates. The only strictly terminal modern fans to have been described in any detail are the Markanda Fan (Mukerji, 1975, 1976; Parkash et al., 1983) and the Gash Fan (Abdullatif, 1989). Unfortunately, both of these have been modified by the cultivation of fan surfaces and the construction of irrigation channels and artificial levees (Parkash et al., 1983; Abdullatif, 1989), and this should be taken into account when using them as analogues for ancient terminal fan systems. Although the Gangetic Plains contain some of the best modern examples of terminal fan systems (Mukerji, 1975, 1976; Parkash et al., 1983; Friend in North et al., 1989), it has been demonstrated that many of the rivers which develop terminal fans are underfit streams occupying larger river valleys that formed during a wetter climatic phase (Mukerji, 1975; Singh, 1987; Singh et al., 1990). In addition, ancient systems often appear to have been much larger than these modern examples (Nichols, 1987). The Medano Creek, Colorado, is an example of a terminal fan which terminates in an aeolian sand sheet environment (Langford and Bracken, 1987; Langford, 1989). This system is rather lira-

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Fig. 1. Map of the terminal fan developed by the Rwer Gash. Sudan (after Abdultatif, 1989; pers. commun., 1992). The Gash Fan is an excellent example of a modern terminal fan system although it has been subjected to modification by human cultivation and irrigation. The feeder, distributa~' and basinal zones discussed in the text are indicated.

ited in lateral extent (the fan measures 1(1 km in length and 7 km in width). The Mojave River Wash, California, is another small terminal fan which terminates partly in aeolian sand sheets and dunes, and partly in playa mudflats (Langford, 1989). Aeolian environments can also form a significant component of larger terminal fan systems; for example, approximately 11% of the surface of the Gash Fan (Fig. 1) comprises aeolian sediments (O. Abdullatif. pers. commun.. 1992). Terminal fans can be subdivided into single entry systems (supplied from a point source) and

!

T E R M I N A L F A N S - - A R E V I E W WITH R E F E R E N C E T O D E V O N I A N E X A M P L E S

multiple entry systems. Only single entry systems have been described in detail from modern settings (e.g. Mukerji, 1975, 1976; Parkash et al., 1983; Abdullatif, 1989). Examples of modern multiple entry systems possibly include the wadi belts and piedmont alluvial plains described from north Africa (Glennie, 1970; G.E. Williams, 1970a). Processes

The two primary processes involved in the development of terminal fans are the break up of a stream into a network, and subsequent water loss through evaporation and infiltration. An understanding of these phenomena is critical in the assessment of such systems.

Channel diversion and bifurcation The cause and mechanism of diversion and bifurcation in terminal fans is frequently unclear, although the ultimate result is always to dissipate the energy of a system. Highly variable or seasonal discharge results in wide fluctuations in the amount of transported sediment load and can result in rapid deposition. Rapid aggradation may choke a channel midstream ("mid-channel overloading" of Leopold and Wolman, 1957) with the result that subsequent flow is divided around the obstruction. If channel threads do not rejoin then the channel may bifurcate (Parkash et al., 1980). Alternatively, diversion and bifurcation may occur via a process analogous to avulsion or crevasse splay development, with turbulence induced scour being concentrated at a particular point on a channel bank. This may result in breaching of the bank or levee and the establishment of a minor distributary channel (Mukerji, 1975). The angle of divergence, the inner angle between the parent and minor distributary measured in the downstream direction, is generally < 90 ° in most distributary systems. Mukerji (1975) observed in the Markanda fan that with increasing distance down-system, the angle of divergence decreases. Experimental data summarized by Garde and Ranga Raju (1978) suggest that as a consequence of the decreasing angles of diver-

341

gence, the proportion of diverted bedload may decrease down-system. Observations on natural channel bifurcations indicate that the diversion of bedload as well as the diversion of suspended load is mainly dependent on the relative transporting capacities of the parent and minor distributary channels (Axelsson, 1967). Mukerji (1975) also observed that the lengths of successive distributary channels increase progressively with increasing distance down-system. This may also be explained by a decrease in angles of channel divergence allowing the diversion of greater discharge volumes into the distributaries and thereby increasing their survival lengths. The effects of diversion on an alluvial stream are related to the hydraulic effects mentioned above and the erodibility of the bed. While a minor distributary channel will often draw a relatively large share of the bedload, the parent channel downstream of a bifurcation will carry relatively less bedload. This may lead to erosion in the parent channel downstream of the diversion, although sediment deficiency may be partially balanced by erosion of channel banks (Garde and Ranga Raju, 1978).

Ecaporation and transmission loss Downstream reduction in discharge is a characteristic feature of terminal fans and is largely due to the combined effects of infiltration and evaporation. Evaporation and evapotranspiration can be important causes of water loss in ephemeral streams, especially if vegetation colonizes ephemeral channel courses (Hellwig, 1973a, b). Data from streams in arid climates such as the Gila (Arizona) and the Swakop (South Africa) indicate evaporation losses of 20-30 m 3 ha 1 day -I from stream beds alone (Culler, 197(I; Hellwig, 1973b). Evaporation on the floodplain is also important and can lead to the local development of salt flats or "salinas". Infiltration into a channel perimeter is termed "transmission loss". Ephemeral streams dominate tropical to subtropical semi-arid and arid drainage networks and are characterized by downstream changes as they lose water to the

342

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surrounding alluvium (Leopold and Miller, 1956; Schumm and Hadley, 1957; Schumm, 1961). Rates of transmission loss are mainly a function of the permeability of the channel perimeter (Simon and Richardson, 1966; FAO, 1981). Transmission losses cause flood peaks and total discharge values to decline in the downstream direction (Babcock and Cushing, 1941: Burkham, 1970). In addition, transmission losses recharge groundwater aquifers (Renard and Keppel, 1966), increase the suspended-load concentration (Leopold and Miller, 1956) and promote aggradation (Schumm and Hadley, 1957; Bull, 1991). The magnitude of transmission losses is influenced by the relationship between inflow volume and the cross-sectional form of the channel. Many ephemeral channels are characterized by rectangular cross-sections with high w i d t h / d e p t h ratios (Schumm, 1961) and consist of an inner "axial zone" of low flow bordered by an "outer zone" which is only active during floods (e.g. Abdultatif, 1989). This means that a small increase in depth during flooding will yield rapid increases in the wetted perimeter with a consequent increase in transmission loss. Transmission losses are often so great in arid and semi-arid zones that eventually most surface flows decline to zero (Graf, 1988). If primary drainage channels of arid and semi-arid zones do

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not terminate at lakes or the sea, they often braid into a complex of micro-channels and are lost (Twidale, 1972; Rust, 1981). It is unusual for the active reach of an ephemeral water-coursc tt, extend more than 100 km downstream of its poini of emergence onto an alluvial plain (FAO, 1981, p. 5). This distance may therefore represent a broad upper limit for the downstream extent ot terminal fan systems. Modern terminal fan systems

A simple model is presented li,~ .,,and dominated and mixed-load tcrminal fans, I'hc model utilizes a subdivision of systems into feeder. distributary and basinal zones. This is largely based on descriptions of modern lcrminat fan systems (e.g. Fig. l), but also incorporates aspects of closely related systems. Each zone is characterized by different discharge regimes (Table' 1!. Feeder zone

The feeder zone or "'inner" fan area is dominated by the main feeder channel(s) and associated interchannel areas (Fig. 1). Although a well developed channel network may exist wilhin the feeder zone, a main channel, carrying a discharge greater than any other channel, can often be identified. This is usually characterized by its

TABLE I Characteristic discharge regime and dominanl facies of each zonal subdivision used in the description ot terminal lan~ Dominant processes

Dominant facies association

eeedt'r Zone

Channel flow > or ~- interchannet (interchannel: sheetflood _+ lacustrine ._+ aeolian)

Single or multistorey channel sandstone/conglomeralc bodies, lt)'s m thick, l()0's m wide. Overbank m u d s t o n e s / ~ a n d s t o n e s ± aeolian sandstones

Distributary zone Proximal: streamflow >> sheetflood

Multistorey channel sandstone bodies,

Medial: streamflow > or -~ sheetflood + aeolian

up 100 m thick (individual storeys < 5 m), 100s m wide Channel sandstone bodies (single and multistoreyL

Distal: sheetflood > streamflow + aeolian

2~5 m thick (generally < 100 m wide) Sheetflood sandstones + floodplain siltstones, depositional units < 2 m thick, 10s to 100s m wide, + aeolian sandstones

Basinal zone Sheetflood _+ aeolian _+ chemical

Floodbasin siltstones + aeolian sandstones _+ playa mudstoncs and cvaporites

T E R M I N A L F A N S - - A R E V I E W WITH R E F E R E N C E T O D E V O N I A N E X A M P L E S

greater width rather than its depth (e.g. Abdullatif, 1989). Feeder channels generally occupy relatively long-lived courses and are often slightly (or even deeply) entrenched. The reoccupation of channels is also more probable in the feeder zones of systems (cf. Rachocki, 1981). The main channel may define the "dynamic axis" of a system (Rachocki, 1981). This axis may gradually migrate across a system or "fan" surface (cf. Kosi River, Gole and Chitale, 1966). Current directions from the main channel at any specific point in the feeder zone will generally show a limited spread (cf. Bluck, 1980). Although the primary channel(s) may be perennial, it is likely that they are subject to dramatic variations in discharge due to marked seasonal variations in precipitation. If sufficiently coarse sediment is available, gravel deposition may characterize the feeder zones. Such gravelly zones or "Bhabar" occur on the Gangetic Plain and extend for 8-24 km in a downstream (Geddes, 1960; Parkash et al., 1980). However, these relatively coarse-grained zones quickly pass downstream into regions dominated by finer-grained deposition, probably accompanied by a decrease in slope. Interchannel areas generally receive little coarse-grained sediment although lateral overbank flooding can produce sandy "splays" and deposit large amounts of mud/silt (Parkash et al., 1983). In mixed-load fluvial systems, interchannel sequences within feeder zones are generally dominated by mudstones and occasional sheetflood sandstones, possibly with local lake deposits. In sand-prone systems aeolian facies may dominate the interchannel environment (e.g. the Great Sand Dunes along the Medano Creek, Colorado, Langford, 1989). Distributary zone

Distributary zones are dominated by distributary channels which are the result of the downstream bifurcation of the main feeder channel(s). Interehannel areas are often limited in extent due to the active nature of the distributary channel courses. Distributary channels are commonly

343

ephemeral and often characterized by features such as low-stage braiding and bar emergence (Abdullatif, 1989). However, perennial, though fluctuating flow may occur in larger distributary channels (Mukerji, 1976). The characteristics of ephemeral distributary channels, particularly channel width, fluctuate spatially and temporally in response to variations in mass and energy input (Stear, 1985; Graf, 1988). The behaviour of an aggrading ephemeral channel will reflect its constant tendency toward dynamic equilibrium and the lagged responses when flows change rapidly (Schumm and Hadley, 1957; Thornes, 1980; Howard, 1982). As a consequence, overall channel morphology is often difficult to predict, although it will probably be related to the scale and duration of the last major flood discharge. Detailed morphology in terms of bar and repetitive bedforms often reflects the falling stage history of scour and fill (Thornes, 1980). Incised distributary channels die out downstream, and flows become progressively less well defined. These more distal areas may be characterized by aggrading fan-like lobes (Mukerji, 1975). Sheetfloods tap the bedload of the distributaries and their deposits are therefore sanddominated (cf. McKee et al., 1967). Finer-grained sediments are deposited at low stage and are also carried beyond distributary lobes at high stage. The erosive capability of sheetfloods is relatively minor (Kirkby and Morgan, 1980) and most of the transport takes place in relatively high velocity threads, possibly related to longitudinal spiral vortices (cf. Olsen, 1989, pp. 217-218). The differentiation of sheetfloods from streamfloods is not always clear (Rahn, 1967; G.E. Williams, 1970b, Hogg, 1982; Graf, 1988) and observations of modern sheetfloods are rare (Davis, 1938; Aldridge and Eychaner, 1984). Like their channelized counterparts, unconfined flows display spatial and temporal variation in their hydraulic characteristics (Hogg, 1982). Consequently, erosion and scouring may or may not occur at the sheetflood base and a sheetflood may terminate in a minor mudflow. After flood events, temporary lakes or terminal " p a n s " (cf. Ward, 1988) may persist for some

344

months, possibly developing ephemeral lake deposits. Prolonged exposure of interchannel areas may on the other hand result in aeolian reworking and development of aeolian sand sheets and dunes (Wopfner and Twidale, 1988; Langford. 1989). Distributary zones of terminal fan systems are thus characterized by a combination of channel, sheetflood and suspension dominated flows and aeolian activity which can result in a complex mixture of deposit types and facies. There is. however, a general tendency for distributary channels to dominate in proximal reaches and to be gradually replaced by sheetfloods down current (Fig. I; Table 1). Deposits and sedimentary structures related to distributary channels and sheetfloods are described by Parkash et al. (1983), Sneh (1983) and Abdullatif (1989) from modern distributary systems. Basinal z o n e

In addition to the active zones of a terminal fan system there is also the region into which the system drains. This area may simply be an alluvial floodbasin (Parkash et al., 1983), or a playa mudflat (Langford, 1989). Alternatively the basinal area may be characterized by aeolian environments (Langford, 1989). The basinal zone will generally only receive very fine-grained sediment after large floods, and distributary channels will extend into this zone only during extreme flood events. Devonian terminal fan systems

There are several limitations related to the study of terminal fan deposits. Firstly, the scale of many characteristic features of terminal fan systems generally exceeds that of a single exposure (Friend, 1978, p. 531). This is further complicated by the frequent inability to accurately correlate between outcrops (Friend, 1978). These problems are discussed further in relation to specific case studies. Additional problems occur when comparing modern and ancient systems, particularly the effect of sediment-binding vegetation on fluvial

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processes (Schumm, 1968, pp. 1583-1584). In many modern semi-arid regions riparian phrcatophytic vegetation often grows in or near river channels (Schumm, 1961: Heltwig, 1973b; GraL 1988). Such plants have tap-root systems thal connect directly with the ground-water table (Robinson, 1958). Because of this relationship a strong interdependency exists between phreato.. phytic vegetation and channel processes (Graf, 1988). Although M i d d l e - U p p e r Devonian land plants were widespread (Allen and Dinelcy. 1988). their ability to bind top-stratum sediment is questionable. This is thought to have been a critical aspect of sedimentation during the Devonian as it will have led to a decrease in the cohesion of channel banks, thereby promoting channel mobility (Friend, 1978). It will also have increased the rate of run-off and ability to transpori sedimenl (Friend, 1978). As a result of these influences ~l controls on depositional style, it is generally noi possible to find precise modern analogues for ancient terminal fan depositional systems (Miall. 1980, p. 711. In the following section, examples of Devonian terminal fan systems from the Easi Greenland Basin and the Munster Basin. southwest Ireland are described, and a general sedimentological model is proposed which incorporates the most important aspects of these ancient systems and relates them to processes within modern terminal fans and related systems. The tripartite subdivision of modern systems is applied to these ancient examples such that sedimentary products are correlated with the relevant discharge characteristics of each zone (Table 1 ). Northeast Greenland Basin

The Devonian Basin of Northeast Gr~renland covers an exposed area of c. 10,000 km :. The basin is oriented N - S and exhibits a sedimentary fill in excess of 8 km thick (Olsen and Larscn. 1993a). The sediments are mainly M i d d l e - U p p e r Devonian fluvial deposits (Friend et al.. 1983; Olsen, 1993). The basin was intensively studied from a sedimentological viewpoint during the late 1960's and early 70's (reviewed by Friend et al.. 1983) and subsequently re-evaluated during the

345

TERMINAL F A N S - - A REVIEW WITH REFERENCE TO DEVONIAN EXAMPLES

late 1980's and early 1990's (Olsen, 1990, 1993; Olsen and Larsen, 1993a, b). Several formations within the basin have the characteristics of largescale terminal fan systems (Friend, 1978; Friend et al., 1983; Olsen, 1993). Two of these systems, the Snehvide Formation and the Rodebjerg Formation, are dealt with in this paper. The terminal fans are single entry, sand-dominated systems terminating in aeolian environments. The two systems share many characteristics in both scale and sedimentary architecture. The exposures available are located in different parts of the

systems. Collectively, however, the two systems form the basis for a composite model of sanddominated terminal fans in the Northeast Greenland Basin. Snehvide Formation

Stratigraphic relationships and general interpretation The Snehvide Formation attains its maximum thickness of c. 250 m at the mountain Snehvide in 23~30' FORMATION

TORBERN ~RjF_ER~BERGMAN

23°30' R U M P E ~ DUSEN FJORC

RODEBJERG FORMATION I 5KM j

sOF~ sUND

Fig. 2. Map of the Middle-Upper Devonian Northeast Greenland Basin. The distribution of formations investigated in this study is illustrated in the two inset maps.

346

~ B. K E L I , ' t

northern Hudson Land, where it occurs sandwiched in the braidplain deposits of the Sofia Sund Formation (Figs. 2 and 3A). At the mountain Torbern Bergman Bjerg in southern Hudson Land, the top and base of the Snehvide Formation interdigitates with the Sofia Sund Formation; the Snehvide Formation wedging out towards the west (Fig. 3A). In western Gauss Halve, west of the dome-shaped fold, the so-called "Moskusoksefjord inlier" (Biitler, 1959), the Snehvide Formation dips down into the Moskusoksefjord in a westward direction. Towards the east, the formation is seen to wedge out into the Sofia Sund Formation (Olsen and Larsen, 1993a). In Moskusokselandet the formation is bounded laterally in the east by volcanic rocks, representing a large volcanic centre (Olsen, 1993). Palaeocurrents in the Snehvide Formation are well defined towards south-southeast at Snehvide and less well defined

towards the west-southwest around Genvejsdalen and Torbern Bergman Bjerg in Moskusokselandet (Fig. 4). The feeder zone is situated at Mt. Snehvide and is composed of conglomeratic feeder channel bodies with subordinate interchannel fines. The distributary zone occurs at and south of Mt. Snehvide. The "proximal" part of the zone is a sandy braidplain succession (upper part of section al Mt. Snehvide), grading downcurrent into the "medial-distal" part with distributa~ channel sandstones embedded in sheetflood sandstones (middle part of succession in Moskusokselandet t. The basinal zone (lower and upper part of succession in Moskusokselandet) is characterized b~ aeolian dune and sand sheet deposits associated with subordinate sheetflood, floodbasin and ephemeral stream deposits. The laterally rc stricted nature of the formation indicates :~

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Fig. 3. (A) Snehvide Formation. Facies association logs illustrating the downstream variation within the terminal fan system. At Mr. Snehvide, feeder zone deposits form the lower part of the succession transitionally overlain by "'proximal" distributa~' zone deposits. At the mountain Torbern B e r g m a n Bjerg " m e d i a l - d i s t a l " distributary zone deposits dominate. The terminal fan system is enveloped in braidplain deposits of the Sofia Sund Formation which exhibits interfingering with aeolian basinal zone deposits at Torbern Bergman Bjerg. (B) Legend for the logged sections in this paper. Facies association key is used only for Figs. 3A and 13A

'ERMINAL FANS--A

REVIEW WITH REFERENCE

TO DEVONIAN

347

EXAMPLES

FACIES ASSOCIATIONS

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SEDIMENTARY STRUCTURES

AEOLIAN IRREGULAR LAMINATION

AEOLIAN HORIZONTAL LAMINATION

AEOLIAN LOW-ANGLE INCLINED LAMINATION

I~~

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WIND RIPPLE FORMSETS

AEOLIAN CROSS-BEDDING

PLANAR CROSS-BEDDING

FESTOON CROSS-BEDDING

TROUGH CROSS-BEDDING

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SCOUR AND FILL BEDDING

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UNDIFFERENTIATED STRUCTURES IN MUDSTONE/SILTSTONE

MISCELLANEOUS FEATURES

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from the north-northwest (Fig. 4). In that direction, c. 20 km from Mt. Snehvide, a narrow halfgraben appears to have developed. This was elongated in a N N W - S S W direction and may haw: been initiated prior to deposition of the Snehvidc Formation (P.-H. Larsen, pets. commun., i99(b, thus forming a narrow valley through which coarse clastics were transported by a gravelly rivet, tn Moskusokselandet, a volcanic centre and 1he "Moskusoksefjord inlier'" formed topographic ~ barriers. The volcanoes forced the fluvial system to turn to the western flank of the "Moskusoksefjord inlier".

bKM Feeder z o n e

Fig. 4. Snehvide Formation. Palaeocurrents from the Snehvide Formation at three localities. Notice the larger spread in palaeocurrent directions in the distributa~, zone at Genvejsdalen. N u m b e r of m e a s u r e m e n t s is indicated at each locality.

point-source and the system should therefore be classified as a single entry system. The palaeocurrent directions at Mt. Snehvide indicate that the coarse sediments were derived

In the most proximal part around Mr. Snchvide the formation comprises a lower portion dominated by conglomerates and an upper portion dominated by pebbly sandstones, Only the lower conglomeratic portion is considered as representing the feeder zone of the sy,qem. The conglomerates occur in channel-shaped units I-4, m thick (Figs. 5 and 6), and are dominated b'¥

Fig. 5. Snehvide Formation, feeder zone deposits. Two feeder channel fills occur interbedded in fine-grained mterchannel sandstones. The lower channel fill is a small and simple type composed entirely of conglomerate with sandstone olfl.v as thiJl lowstage deposits. The upper channel fill represents the main channel in the feeder zone. It is thicker and wider and composed o f , lower conglomeratic part (channel axis) and an upper part of pebbly sandstone (outer channel zone). See also Fig. 6. Nt~te rucksack for scale.

TERMINAl.

FANS--A

REVIEW

WITH REFERENCE

TO DEVONIAN

1O's OF METRE~

Fig. 6. Snehvide Formation. Diagrams illustrating the two types of channel fills found within the feeder zone. The large, complex type represents the main feeder channel.

scour-and-fill structures, horizontal lamination and massive bedding (Fig. 7). The scour-fills are interpreted as trough-fills associated with the migration of gravel dunes (e.g. Rust, 1978, 1979), although the coarse grain size has resulted in poor foreset definition. The horizontal lamination and massive bedding are interpreted as the products of longitudinal bars (cf. Smith, 1974). Two main types of feeder channel fill occur in these sediments. The smaller type is < 3 m thick, tens of metres in lateral extent (Fig. 6) and is entirely composed of conglomerates except for thin, sandy low-stage deposits. The larger type is 3 - 6 m thick, hundreds of metres wide, and is composed of conglomerate in the deep central portion with pebbly trough cross-bedded sandstones in the shallow lateral parts of the channel and in the top part of the central channel fill (Fig. 6). These two types of feeder channel fill reflect the co-existence of relatively small, simple channels and larger more complex channels. The latter were characterized by a deeper central zone with gravel transport and shallow lateral parts typically carrying migrating sandy dunes. In one exceptionally well exposed outcrop, perpendicular to the current direction, a convex-up morphology of the pebbly sandstone unit in the shallow lateral part was observed. Fine-grained sandstones occupied the space between the cross-bedded pebbly sandstone unit and the erosive channel margin. This example suggests that the sandy dunes (pebbly sandstones) migrated on large braid bar complexes (cf. Ashley, 1990) in the lateral part of the channels. Between the feeder channel bodies, very fine and fine-grained pebble-free sandstones occur, in units up to 5 m thick. The sandstones are dominated by parallel lamination and cross-lamination, associated with low-angle festoon cross-bed-

EXAMPLES

349

ding (Fig. 7). Mudstones also locally occur. Channel features are lacking in these fine-grained deposits, which suggests deposition from unconfined flows. The distinct separation between channel-shaped conglomerates and fine-grained deposits suggests that the fine-grained deposits accumulated at flood stage, when otherwise inactive high-level tracts or interchannel areas were inundated. The characteristics of the large feeder channel fills resemble the model for distal gravelly braided rivers presented by Rust (1978, 1979), based mainly on the middle reach of the Donjek River. The middle reaches of the Donjek River are characterized by a deeper axial part with gravels and shallow lateral parts with sandy bedload. Inactive tracts are mainly subject to deposition of mud due to abundant vegetation, unlike this ancient example. The subdivision of the channels in the axial and outer zone also finds an analogue in the feeder channel of the Gash Fan (cf. Abdullatif, 1989). The four sections shown in Fig. 7 illustrate the downcurrent variations in sediment characteristics along a 3 km stretch approximately parallel to the palaeocurrents (Figs. 4 and 8). Individual channel fills clearly decrease in thickness downcurrent and there is a corresponding decrease in maximum pebble size (MPS). The abundance of feeder channel bodies also decreases downcurrent. These deposits are gradually replaced by sandy braidplain deposits. In the conglomerates, scour-and-fill bedding and trough cross-bedding are replaced by horizontal lamination and massive bedding in a downcurrent direction. A few hundred metres downcurrent of the most distal section in Fig. 7, the feeder channel bodies with associated overbank deposits are replaced entirely by braidplain deposits. In conclusion, the feeder zone was composed of well defined gravelly channels and associated overbank areas which graded downcurrent into a broad sandy braidplain ("proximal" distributary zone; see below). The thickness of the feeder channel units suggests that the gravelly channels were relatively deep (up to 6 m) in the proximal part of the feeder zone but decreased to 2-3 m in the distal part of the zone. This decrease was

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combined with a general change from gravelly dunes to longitudinal bars as the dominant bedform.

Distributary zone

Downcurrent and up section of the feeder zone succession at Mt. Snehvide, medium-grained pebbly sandstones with subordinate fine pebble conglomerates occur (Fig. 7). The sandstones are mainly trough cross-bedded, but horizontal lamination is common, interbedded on a metre-scale. The conglomerates exhibit scour-and-fill bedding and trough cross-bedding and occur as decimetre-scale beds within the sandstones. The interbedding of facies is apparently unsystematic. The conglomerates decrease in abundance from c. 40% in the most upcurrent section to < 5% in the section 3 km downstream.

351

These sediments are interpreted in terms of shallow braided channels in which sand dunes dominated, with gravel dunes in the deeper parts of channels and upper plane beds in shallow parts. In contrast to the gravelly braided channels, these sandy braided channels are not associated with finer-grained overbank deposits. This suggests a braidplain origin for the sediments of the "proximal" distributary zone. Approximately 15-20 km south of the Snehvide locality, in Moskusokselandet at the mountain Torbern Bergman Bjerg and around Genvejsdalen (Figs. 2 and 3), the main body of the Snehvide Formation comprises very fine to finegrained sandstones (c. 75%) and 2-5 m thick fine-to medium-grained sandstone units (c. 20%). Both components are of fluvial origin, and arc associated with decimetre-scale fine-grained sandstone beds of aeolian dune and sand sheet origin (Fig. 9A).

Fig. 8. Vertical aerial photograph of the Snehvide Formation exposed on the Mt. Snehvide in H u d s o n Land (Fig. 2). The positions of the sections in Fig. 7 are indicated. Photo courtesy (route 872A, no. 55) of Kort-og Matrikelstyrelsen. D e n m a r k (permission A200/87).

352

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The very fine to fine-grained sandstones are characterized by the dominance of parallel lamination, which may be horizontal, low-angle or represent the concordant infilling of shalto~. troughs (Fig. 10). The lamination is associated with parting lineation. Individual sedimentation units in these deposits are difficult to define duc to the lack of irregular erosion surfaces. However, aeolian interbeds, desiccation cracks and wave ripples on parting planes indicate that sedimentation units are probably of the order of ] m thick or less. These deposits are interpreted as the products of sheetfloods, with flow conditions around the transition between u p p e r and lower flow regime (cf. Shaw, 1972; Saunderson and Lockett, 1983; D a m and Andreasen, 1990). The fine- to medium-grained sandstone units are mostly trough cross-bedded and composed of 1-3 storeys. No signs of emergence occur within the units. This may, however, be a consequence of low preservation potential. They were probably deposited in low sinuosity, braided channels characterized by ephemeral or fluctuating perennial flow and channel depths of the order of 2 m. In conclusion, the main body of the formation in Moskusokselandet is interpreted as a system of distributary channels with extensive sheetflood areas. The latter were inundated during floods and temporarily exposed to aeolian rcworking. The succession represents the "medial--distal" part of the distributary zone. Basinal zone

The lower and u p p e r parts of the formation in Moskusokselandet around Torbern Bergman Bjerg are characterized by a dominance of aeolian dune and sand sheet deposits (c. 80%) with subordinate sheetflood, ephemeral stream and

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Fig. 9. Snehvide Formation at the valley Genvejsdalen (Fig. 4). (A) Log through a sequence developed within the "'medial-distal" distributary zone. Distributary channel deposits are interbedded with sheetflood deposits and rare aeoliat~ dune ( A D ) deposits. (B) Basinal deposits dominated by aeolian sediments. A D = aeolian dune; FB = flood basin; ES = ephemeral stream deposits. Notice the different scale compared to (A).

VERMINAL

FANS--A

REVIEW

WITH REFERENCE

TO DEVONIAN

353

EXAMPLES

Fig. 10. Snehvide Formation at the Genvejsdalen locality. Parallel laminated fine-grained sheetflood sandstones of the "medialdistal" distributary zone.

flood basin deposits (Fig. 9B). These sediments probably represent the basinal part of the system, with abundant aeolian deposition and only occasional inundation by extreme floods.

Rodebjerg Formation

Fjord, the limited lateral extent of the formation and the increasing dominance of aeolian deposits in the downcurrent direction collectively suggest that the Rodebjerg Formation was a single entry ]

Stratigraphic relationships and general interpretation The Rodebjerg Formation is exposed on the islands of Ymer O, Geographical Society O and Traill O (Fig. 2). The formation was studied in detail on Ymer 0 at the mountains Rumpen, Angelin Bjerg and Rodebjerg, and at the valley junction of Jfilut Dal/Fladedal. The formation is dominated by fluvial sandstones in the northern outcrops grading southwards into aeolian sandstones. Palaeocurrents in the Rcdebjerg Formation are directed towards the southeast, palaeowinds are from the south (Fig. 11). Reconnaissance studies of the formation south of Ymer O show a dominance of aeolian dune deposits and indicate that the formation wedges out in an eastward direction on Traill O (Olsen and Larsen, 1993a). The absence of the formation north of Dus6n

Fig. 11. Rodebjerg Formation. Palaeocurrent rose diagrams. Black = fluvial; white = aeolian. Number of measurements is indicated. Notice the large spread in fluvial current directions in the "distal" distributary zone at R0debjerg.

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355

T E R M I N A L F A N S - - A R E V I E W WITH R E F E R E N C E T O D E V O N I A N E X A M P L E S

T h e s e d i m e n t s e x p o s e d southeast of R u m p e n and on A n g e l i n Bjerg are c o m p o s e d of multistorey distributary channel sandstones and sheetflood sandstones with rare aeolian sandstones. T h e y are interpreted to represent the "medial"

terminal fan system. The formation rests on braidplain deposits of the Sofia Sund Formation on Ymer ~ (Olsen, 1993), and the terminal fan system is interpreted as being surrounded by this braidplain system of perennial rivers.

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hg. 14. Rodebjerg Formation, "distal" distributary zone. (A) General field appeaJ~wc dlustrating the thin beds of constituent facies associations. (B) Ephemeral distributar~, ,:hannel deposits. Pencil for scale. (C) Sheetflood deposits, 10 cm scale bar in upper right, (D) Aeolian sand sheet deposits. Translatent stratification is seen in the upper and lower part. Irregular adhesion lamination l~ ~ccn in the central part. ll)-cm bar for scale.

A

TERMINAL

FANS--A

REVIEW

WITH REFERENCE

TO DEVONIAN

EXAMPLES

part of the distributary zone. Current directions occurred within a relatively narrow range (Fig. 11). The sediments at Mt. Rodebjerg (except the basal part) are dominated by minor distributary channel and sheetflood deposits associated with multistorey distributary channel sandstones, flood basin siltstones and aeolian sandstones. This sequence is interpreted as the distal part of the distributary zone where ephemeral flow was dominant and aeolian reworking common. Current directions from the variety of channelized streams and sheetfloods are highly variable (Fig. 11). The thick basal part of the Rodebjerg succession is dominated by aeolian sand sheet deposits and represents the basinal tract of the terminal fan system. Aeolian dune deposits are dominant in the Jfilut D a l / F l a d e d a l area. Similar deposits occur on Geographical Society ~ (Fig. 2) and they are probably characteristic of the basinal portion of the system further south. The feeder zone is not exposed in the present day outcrops. A large feeder channel may have entered the basin through a valley, represented by W N W - E S E trending normal faults around Noa Dal (western continuation of Dus6n Fjord, see Larsen and Bengaard, 1991; Fig. ll). The fact that the Rodebjerg Formation is not present north of Dus~n Fjord (Fig. 2) seems to support this suggestion. Distributary zone

The northern outcrops, southeast of Rumpen and on Angelin Bjerg, are characterized by 5-10 m thick and > 100 m wide multistorey bodies of fine- to medium-grained sandstone alternating with 0.5-2 m thick units of very fine to finegrained sandstones (Fig. 12). Both are of fluvial origin and are associated with rare aeolian dune interbeds. The sediments resemble the " m e d i a l distal" distributary zone of the Snehvide Formation, exposed in Moskusokselandet (compare with Fig. 6) and are likewise interpreted as the deposits of braided distributary channels, sheetfloods and aeolian reworking. The channel bodies are, however, thicker and comprise more than 80% of these sequences. Individual storeys range

357

in thickness from 1.5 to c. 4 m, probably corresponding to local channel depth. Relatively largescale trough to wedge-shaped cross-sets are common. They tend to occur in the lower part of storeys. Occasionally downcurrent dipping internal bounding surfaces are observed, associated with descending cross-bedding (Fig. 12B). This probably indicates braid bar complexes with descending dunes, growing in size as they entered deeper water in the lee of the macroforms (cf. Banks, 1973; Haszeldine, 1983a, b). The sheetflood deposits are entirely composed of parallel laminated sandstones and form only c. 10-15% of the formation in these outcrops. Individual flood units are difficult to differentiate due to lack of irregular downcutting. Aeolian dune deposits form less than 5% and occur in metre-scale beds. The R u m p e n / A n g e l i n Bjerg sequences are interpreted to represent the "medial" distributary zone with braided distributary channels associated with sheetflood-dominated plains and local aeolian dunes. Downcurrent, at Mt. R0debjerg a succession of fluvial, aeolian and floodbasin deposits occurs (Figs. 13A and 14A). The fluvial sediments are dominant, except in the lower 200 m of the formation, and comprise channelized ephemeral stream, sheetflood and braided channel deposits. Trough cross-bedded fine- to medium-grained sandstone is the dominant facies of the ephemeral stream deposits (Figs. 13B, 13C and 14B). These sandstones generally occur as single storey sheetlike ( < 100 m wide) channel bodies less than 1 m thick which commonly fine upwards. The sheetflood deposits are dominated by very fine to medium-grained parallel laminated, cross-laminated and massive sandstones in decimetre-scale beds (Fig. 14C). Intervals up to 40 m thick are similar to the northern outcrops, being dominated by trough cross-bedded sandy braided channel deposits (Fig. 13E). The fluvial sandstones are closely associated with siltstones of floodbasin origin (Fig. 13B, 13C and 13E) and sandstones of aeolian sand sheet and (minor) aeolian dune origin (Figs. 13D and 14D). This succession (excluding the basal 200 m) is interpreted as the "distal" part of the distributary zone.

358

%.B. KEE.LY A N I ) H I,)[.SI:.N

Fig. 15. Rodebjerg Formation, basinal deposits. Aeolian sediments, exposed in the Jfilut Dal gorge. A large sample trough cmss-sel is observed. Palaeowind obliquely into the exposure.

inated, with translatent strata (cf. Hunter. 1977) and irregular lamination of adhesion ripple origin, indicating alternating dry and damp surface conditions (Olsen, 1993). Ephemeral stream deposits form only c. 5% of the basal part. These

Basinal zone

The basal 200 m of the Rodebjerg Formation at Mt. Rodebjerg is dominated by aeolian sand sheet deposits. These sandstones are parallel lam-

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Fig. 16. Schematic cross-section through the Middle-Upper Devonian succession of western part of the Munster Basin, southwest Ireland. S S F = the Sherkin Sandstone Formation; C S F = the Chloritic Sandstone Formation: P S F = the Purple Sandstone Formation. Arrows on inset map indicate general transport directions.

35~t

TERMINAL FANS--A REVIEW WITH REFERENCE TO DEVONIAN EXAMPLES

sediments probably formed in the proximity of the distributary zone where shallow ground water flow frequently resulted in damp surface conditions. At the Jfilut Dal/Fladedal valley junction the formation is composed almost entirely of aeolian sediments (Fig. 15). These deposits correlate with the basal deposits at Mt. R0debjerg (Olsen and Larsen, 1993a; Olsen, 1993). The sediments are dominated by large-scale dune cross-bedding with simple sets up to 10-15 m thick which are laterally associated with cosets of smaller scale trough sets. Such cross-bedded units alternate with 1-2 m thick parallel laminated and medium-scale cross-bedded sandstones, deposited in interdune areas. The sequence is interpreted in terms of a dune field with compound dunes comparable with the Algodones dune field in California (Havholm and Kocurek, 1988). The sediments were probably deposited in a more distal setting than the basal Mt. R0debjerg succession. Munster Basin, SW Ireland

The Munster Basin of SW Ireland lies within the Variscan Orogen and presently outcrops over

an area in excess of 12,500 km 2 (Fig. 16). The basin is oriented WSW-ENE and was the site of prolonged alluvial deposition during Middle-Upper Devonian times with an observed sediment pile thickness of over 6 km (Graham, 1983; Williams et al., 1989). Several formations within the Munster Basin have the characteristics of large terminal fans (Graham, 1983; Williams et al., 1989). Although some detritus was derived locally from the basin margins, sandstone petrography suggests generally distal sources (Graham, 1983). The deposits of two large, broadly coeval, "mixed-load" terminal fan systems have been identified in the western part of the Munster Basin; these are the Chloritic Sandstone Formation and the Sherkin Sandstone Formation (Fig. 16). These systems prograded into "terminal floodbasin" areas which were characterized by the deposition of fine-grained sediment (Valentia Slate, Bird Hill, Caha Mountain and Castlehaven formations). The Chloritic Sandstone represents a transverse system that prograded from the north, whereas the Sherkin system was axial, prograding from the west (Fig. 16). Aspects of the stratigraphy and sedimentology of these units have 0

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Chloritic Sst GLENFLESK

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Fig. 17. Location map of main study sections used to assess the Chloritic Sandstone Formation. Note dominance of southerly directed palaeocurrents (transverse drainage).

360

been discussed by Graham (1983), Kelly (1988a and b, 1992, 1993) and Williams et al. (1989). A third terminal fan system, the Purple SandstoneGun Point Formation is dealt with elsewhere (Sadler and Kelly, 1993). The two fluvial systems share many characteristics on both a gross and detailed scale. It is uncertain whether the systems were single- or multi-entry systems. However, considering the size of the systems (estimated palinspastically restored "radii" of approximately 100 km, Williams et al., 1989; Fig. 4) suggests that they developed from more than one feeder system. This is supported by the regional palaeocurrents which do not appear to radiate (Williams et aI., 19891 in the manner described by Nichols (1987). Although many of the observations and conclusions concerning these deposits are based on a limited number of thick vertical profiles, they are substantiated by proximal-distal observations (cf. Friend, 1978, pp. 531-533; Friend et al., 1983, pp. 4l -44). Chloritic Sandstone Formation

Stratigraphic relationships and general interpretation Recent detailed mapping indicates that this formation locally attains a stratigraphic thickness in excess of 3800 m (P. Meere, pers. commun., 1989). The Chloritic Sandstone Formation is best observed near the Killarney-Mallow Fault in the region of the Derrynasaggart Mountains and the Macgillicuddy's Reeks. The main section used in this study is located between Glenflesk village and Derreenacullig, Co. Kerry (Fig. 171, The Chloritic Sandstone Formation passes laterally and vertically into the Caha Mountain, Bird Hill,, and Valentia Slate formations which are its basinal equivalents. Although there is a gradation between feeder zone, distributary zone and basinal zone associations, it is apparent that the Chloritic Sandstone Formation is dominated by the channelized elements of a distributary system. Palaeocurrents indicate southerly inclined slopes (Walsh, t968;

s.B. K E L L Y A N O H. ~IlAI, N

Graham, 1983; Kelly 1988a; Williams et al., 1989; Fig. 171. Proximal drainage was locally diverted by volcanic centres (Avison, 1984; Kelly, 1988a). In sections located 311-4(t km down-basin (palinspastically restored distance) the Chloritic Sandstone Formation is reduced in thickness to approximately 670 m (O'Sullivan, 1987; Williams et al., 1989) and is composed of approximately 55% sandstone (O'Sullivan, 1987), compared with approximately 72% determined for more ' proximal" sections. The downbasin thinning and fining are clear indicators of the distributary nature of system (cf. Williams et at., 19891.

Feeder zone Within the feeder zone, channel deposits arc composed of individual storeys 5-12 m thick which are generally stacked into multistorey sequences 15-60 m thick. These medium- to coarse-grained multistorey channel sandstone bodies alternate with siltstone units 3-11 m thick. Internally the sandstone bodies exhibit a wide variety of cross-bedding types, although they arc most commonly dominated by sets of medium- to large-scale trough cross-strata (Fig. 19D), Subordinate forms of cross-strata include large-scale tabular sets up to 2.8 m thick. Also present arc cosets of laterally extensive parallel lamination up to 6 m thick (Fig. 19A and 19B). Laminae within these cosets are inclined at angles up to 20". although generally not steep enough to have formed by avalanche processes. The dip direction of the plane beds is often at high angles ( > 45 °) to the local transport azimuth determined from trough cross-strata. The major sandstone bodies are interpreted as products of large braided rivers (Graham. 1983: Kelly, 1988a; Williams et al., 19891 representing the main feeder channels of the terminal fan system. The prominent bedforms of these rivers were dunes which produced sets of trough crossstrata. Less common bedforms included features similar to the "linguoid sandwaves" described by Blodgett and Stanley (1980) which produced tabular sets of large-scale cross-strata. The cosets of parallel lamination are interpreted as representing "sandflats" which grew by a combination of

TERMINAL

FANS--A

REVIEW

WITH

REFERENCE

TO

DEVONIAN

both forward and lateral accretion (cf. Coleman, 1969; Cant and Walker, 1978; Bristow, 1987). The "interchannel" facies association is dominated by thick sequences of massive or weakly laminated siltstones (Fig. 19C). Disseminated calcareous nodules (incipient calcretes) are locally developed. Sandstone units are generally rare and comprise thin, sheet-like beds which are dominated internally by parallel lamination, cross-lamination and/or a solitary set of crossstrata. Individual sandstone beds often fine upwards into siltstone. The general sheet-like form of interchannel units indicates depositional surfaces with little relief. The occurrence of polygonal mudcracks

361

EXAMPLES

and incipient calcretes implies desiccation associated with subaerial exposure. Deposition was mainly from decelerating overbank flows which spread laterally over the floodplain, eventually ponding with the deposition of fine suspended sediment. The apparent lack of stratification within many siltstone units probably results from an absence of any pronounced lithological contrast and/or bioturbation. Sand was episodically introduced into interchannel areas in the form of sheetfloods and sandy splays similar to those described by O'Brien and Wells (1986). The environmental setting of the interchannel facies is possibly analogous to the "intercone" areas described by Geddes (1960) which occur Fir

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362

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D i s t r i b u t a r y c h a n n e l s a n d s t o n e bodies are fine-

f i n e - g r a i n e d s e d i m e n t s (Parkash et al., 1980).

to m e d i u m - g r a i n e d a n d generally 2 - 4 m thick. T h e s e may be a m a l g a m a t e d with finer-grained

Distributary zone

parallel l a m i n a t e d s a n d s t o n e to form multistore~ units u p to 20 m thick. C h a n n e l fills are domi-

I n the d i s t r i b u t a r y zone, c h a n n e l s a n d s t o n e

n a t e d by small- to m e d i u m - s c a l e sets of trough

bodies occur t o g e t h e r with f l o o d b a s i n a n d sheet-

cross-strata (Fig. 19D). A l t h o u g h lateral exposure

Fig. 19. Chloritic Sandstone Formation. (A) Feeder zone--channel and interchannel facies associations, Glenflesk Valley. A thick channel sandstone body rests on interchannel sittstones. Top of cliff face is poorly exposed. Large scale low-angle cross-~trata (e.g. set indicated S) thought to represent the accretion or migration of a large "sandflat". The cliff face is oriented perpendicularly t~ the local transport direction as determined from smaller scale cross-strata. See (B/ for a sketch of the exposure, t(') Fcettc~ zone--interchannel facies association, Glenflesk Valley. Interbedded siltstones and sandstones. (D) Distributary zone, west coast of lveragh Peninsula. Distributary channel sandstone overlyingfloodbasin siltstones. Note rucksack for ~calc

tERMINAL FANS--A REVIEW WITH REFERENCE TO DEVONIAN EXAMPLES

363

Fig. 19 ( c o n t i n u e d ) .

is generally limited, the distributary channel sandstone bodies appear to be broadly lenticular in cross-section ( < 100 m wide), although sandstone body bases generally display little downcutting (cf. Friend, 1978, p. 536). Many units have sharp tops and are directly overlain by siltstone, although some display a fining upward motif with cross-strata being overlain by parallel lamination a n d / o r ripple cross-lamination. The distributary channels were probably formed by relatively small, high-energy streams which often carried dune fields. Increases in flow

strength a n d / o r a decrease in mean grain size resulted in a transition to upper-stage plane beds. The lack of evidence for lateral accretion surfaces and the limited dispersal of palaeocurrent patterns suggests that the streams were of limited sinuosity. Sheetflood and floodbasin deposits within the more distal sequences are characterized by thick siltstones together with 0.5-2.0 m thick units of parallel laminated or ripple cross-laminated finegrained sandstone. Sandstone beds are sheet-like and can often be traced for several hundreds of

364

~ 1'~ K E I + I

metres where exposure allows. The predominance of siltstone suggests generally quiescent conditions punctuated by episodic unconfined flood events which introduced coarser material. These distal sequences are considered transitional to the basinal zone.

Basinal zone The distributary zone of the Chloritic Sandstone Formation exhibits a gradual downcurrent transition to the more basinal deposits of the Caha Mountain, Bird Hill and Valentia Slate formations which are dominated by monotonous sequences of thinly bedded fine-grained sandstone and siltstone (Sadler, 1992). Beds are generally sheet-like with parallel lamination and ripple cross-lamination. The basinal sequences were deposited as flood basin and sheetflood deposits.

--------1 Formation Clltiehlven

~

[

i

:

OtSl.~.+

Sherkin Sandstone Formation

Stratigraphic relationships and general interpretation The Sherkin Sandstone Formation outcrops in the region of Roaringwater Bay with the most extensive exposures occurring on Sherkin and Clear Islands (Fig. 20). The main section used for assessing the Sherkin Sandstone Formation is located on the western side of the South t-larbott~ on Clear Island. The formation is in excess ot 1100 m thick and is dominated by fine- t,+ medium-grained grey-green sandstones interbedded with grey-green and, or purple siltstones. The Sherkin Sandstone Formation passes laterally and vertically into the "Castlehaven Formation" (Re~ illy and Graham, 1976) which is its basinal equiwtlent.

t

Sherkin

FoIIcoagh

Bay Beds ryT,:+:-:.:.;-:I

~r A N ~ ) F [

N

Formation

L J 175

/ /

/

/

WILES0 ~+t OUE TAtS 0

I

"

1

' ~ , ~ HARBO4J~

Fig. 20. Sherkin Sandstone Formation. Location m a p illustrating main study section located on Clear Island (after G r a h a m anti Reilly, 1972). Note the dominance of a large-scale antiform, the Rosscarberry Anticline. Palaeocurrents indicate easterly directed flow (axial drainage).

TERMINAL

FANS--A

REVIEW

WITH REFERENCE

TO DEVONIAN

The nature of the geological structure of the Roaringwater Bay area, particularly the plunging character of the Rosscarberry Anticline, renders proximal-distal relationships of the Sherkin Sandstone difficult to assess. Such relationships are mainly interpreted from the study of the South Harbour Section which clearly documents the "retrogradation" of the system with time. The South Harbour Section can be split into "lower", m

m

365

EXAMPLES

"middle" and "upper" intervals, all of approximately equal thickness (c. 300 m). The lower, basal section is the coarsest with multistorey distributary sandstone bodies (> 5 m thick) being dominant (40%), the remainder comprising minor sandstone bodies of both distributary channel and sheetflood origin (28%), and siltstone (32%). The middle section displays an increase in siltstone content (46%) together with an increase in the m

nl

26" 25

10lO

20 2o

15

15-

0,

A

B

Fig. 21. Sherkin Sandstone Formation. Examples of logged sections. (A) Distributary zone--"proximal" facies association, South Harbour section, Clear Island. Dominated by cross-bedded distributary channel sandstones. (B) Distributary zone--"medial" facies association, South Harbour section, Clear Island. Dominated by cross-bedded distributary channel deposits, together with parallel and ripple cross-laminated sheetflood sandstones and floodbasin siltstones.

366

number of purple lithologies. Multistorey distributary channel sandstones constitute 10% and minor distributary channel and sheetflood units comprise the remainder. The upper section is dominated by fine-grained deposits (siltstone content of 52%) together with minor distributary channels (24%) and sheetflood sandstones (24%). The "lower", "middle" and " u p p e r " sections are thought to relate to the "proximal", "'medial" and "distal" portions of the distributary zone of the Sherkin system. Fieldwork in the area of Roaringwater Bay indicates that the South Harbour Section is representative of the formation and that the vertical changes in depositional style reflect the behaviour of the system as a whole. D&tributary zone

No distinctive feeder zone facies has been recognized within the Sherkin system. The most proximal deposits comprise fine- to mediumgrained, single and multistorey sandstone bodies, the latter tending to be more common (Fig. 21A). Over limited lateral exposures the sandbodies appear to be tabular or sheet-like in form, although deeply incised channels do occur occasionally (Graham and Reilly, 1972, p. 286). Multistorey complexes up to 30 m thick are partitioned into 2-5 m thick storeys by internal erosion surfaces which often display some degree of concave-up downcutting. The predominant sedimentary structure within sandstone bodies is trough cross-stratification, with larger but less common sets of tabular cross-strata (up to 2 m thick). In some instances, sets of cross-strata can be observed to descend in a downcurrent manner with set boundaries inclined at 10-15 ° in the same direction as the foresets. It is also quite common to observe plane beds which "roll over" into sigmoidal foresets. Other sedimentary features such as reactivation surfaces and fine-grained drapes on foresets have also been observed. Lenticular developments of grey-green siltstone, generally less than 3.0 m wide and 0.5 m thick, are quite common within sandstone complexes. The sandstone bodies are interpreted as the products of low-sinuosity streams (cf. Graham and Reilly, 1972; Kelly, 1988b) forming the dis-

",

B K E I 3 Y A N D ~:E.()LS,~N

tributary channels of the terminal fan system. The general lack of pronounced incision reflects broad, relatively shallow channels often cut into a sandy substrate. Bedload within the channels was transported mainly as dunes. Planar .sets of cross-strata represent larger mesoforms similar tc~ those described from modern low-sinuosity sys-terns (Smith, 1970, 1971: G.E. Williams, 1971). The down-current descending sets indicate deposition on the leeside of a larger bedform ~r macroform (Banks, 1973; Miall, 1988). Reactivation surfaces and siltstone drapes suggest a fluctuating discharge. Topset preservation and sigmoidal cross-strata are the result of high-stage bedforms which are developed in the dune tt~ upper-stage plane-bed transition (Roe, 1987). Lenticular "in-channel" siltstones probably reflect the local abandonment of channel segments and deposition from suspension at low stage. Channel depths are estimated to have been generally less than 5 m judging from the scale ol sandstone bodies and sedimentary structures contained within them (Kelly, t992). Within "proximal" sequences of the distributary zone several varieties of fine-grained facies are recognized. Most commonly developed arc sheet-like units of sandstone and siltstone, the former being dominated by parallel lamination or ripple cross-lamination. These units can be attributed mainly to sheetflood processes resulting from channel flooding. A less common interchannel facies consists of dark grey laminated siltstones, light grey bioturbated, pyritiferous siltstone and fine-grained sandstones with wave ripples (e.g. the "Foilcoagh Beds", Graham and Reilly, 1972). These sediments are interpreted as lacustrine deposits on the basis of their fine-grained character, dark colour and the presence of wave generated structures (cf. Clayton and Graham, 1974). A possibtc modern analogue for the Foilcoagh Beds are the "lagoon" deposits of the Lake Eyre Basin. The so-called lagoons are a type of playa that develop in local topographic depressions which are fed by major rivers. Their surfaces are black or grey owing to a cover of organic material (Twidale, 1972), which also could explain the presence of pyrite in the ancient deposits.

TERMINAL FANS--A REVIEW WITH REFERENCE TO DEVONIAN EXAMPLES

The fine- to medium-grained sandstone bodies of the " m e d i a l " and "distal" portions of the distributary zone are generally < 3 m thick (Fig. 22). Medium-scale cross-bedding and parallel lamination (which may exhibit primary current lineation) are the most common sedimentary structures. Thin siltstone units generally < 10 cm thick occur locally within the sandstone bodies. Individual sandstone bodies are generally sheetlike on an outcrop scale (up to several 100 m) (Fig. 22). Gradual downcutting is often apparent in extensive outcrops of thicker units although maximum relief rarely exceeds 2 m. Smaller sandstone bodies ( < 1 m thick) are often dominated by a single set of cross-strata; such units are often stacked up separated by thin siltstones. It is often possible to trace sandstone beds laterally from their more channelized parts. Small- to mediumscale cross-strata pass through plane-beds with primary current lineation to ripple cross-laminae, and eventually the sandstones pass into siltstones which are the finest deposits of distributary zone sequences. These observed lateral transitions

367

clearly indicate flow dissipation across the floodplain. Siltstones are grey-green or purple in colour; individual beds are generally < 1 m thick although they may be stacked to form units up to 20 m thick. Units often appear to be massive or faintly laminated; bioturbation is often conspicuous. Sand-filled polygons and incipient calcrete nodules have been observed, the latter being restricted to purple lithologies. The thicker sandstone bodies dominated by cross-strata are interpreted as larger channel fills. Many of the channels in the distributary zone were evidently small and were filled quickly by dune bedforms. The cross-bedded sheet sandstones probably formed reflect laterally unconfined to broadly channelized sheetfloods which dissipated rapidly, with mud drapes deposited at falling/low stage (Tunbridge, 1981; O'Brien and Wells, 1986). The formative streams were generally 1-2 m deep judging from the sandstone body thicknesses. Other channels which exhibit 3 - 5 m fining and thinning sequences were probably filled

Fig. 22. Sherkin Sandstone Formation. Distributary zone--"medial" and "distal" facies associations. The section isJapproximately 15 m thick. The deposits are mainly sheetflood sandstones and interbedded floodbasin siltstones. Note the dominance of sheet-like geometries with minimal downcutting. The photograph has rotated from the horizontal to account for the steep structural dip.

368

s B K[:l,l."t A N D H ~)1,~1:,",

episodically with increments of sand deposition separated by thinner silts which accumulated during more quiescent phases (cf. Nichols, 1987).

short-lived rejuvenation of the Sherkin Sandstone system. A model

Basinal zone

The commonality of ancient terminal fan systems has been noted by Friend (1978, p. 531). Many units are thick (hundreds to thousands o/( metres) and accumulated in relatively small basins < 105 km 2) near to uplifting mountain sources. Terminal fan sequences are most readily distinguished from other fluvial systems by consistent downstream trends in various sedimentary parameters. The grain size and scale of individual channel sandstone bodies will show a general decrease downstream (Friend, 1978). This is corn-

The distributary zone exhibits a gradual downcurrent and upsection transition into the Castlehaven Formation, which is the "basinal" equivalent of the Sherkin system. The Castlehaven Formation is dominated by purple siltstones with minor ripple laminated fine-grained sandstone interbeds (Graham and Reilly, 1972; Reilly and Graham, 1976). Coarser sandbodies are rare and generally limited to the "Ballylinchy Tower Member" which presumably represents a relatively

Facies associations

Feeder :~<~3-o~ ch.... I

~ ~

Distributary ch.... I

~

Sheetflood

~ , r ~ u d a b ~ ....

Fig. 23. Facies model for terminal fans, ( ] = feeder zone; (,2-4)= distributary zone, shown as "proximal" (2), "'medial" (3) and "distal" (4); (5) = basinal zone. "['he proportions of facies associations may vary in accordance to the suspended Ioad/bedload ratio. The maximum down stream extend of individual terminal fan systems is unlikely to exceed 100 k m

TERMINAL F A N S - - A REVIEW WITH REFERENCE TO DEVONIAN EXAMPLES

bined with an overall increase in the proportion of siltstone if the system carries a mixed load. The simple model presented below (Fig. 23; Table 1) summarizes the facies and achitecture of terminal fan deposits and is based on the observations of modern and ancient systems. Feeder zone

The feeder zone is characterized by two distinctly different facies associations. Dominant is the "feeder channel association", which is composed of individual storeys measuring a few to several metres in thickness. The storeys may be stacked into multistorey sediment bodies measuring tens of metres in thickness and hundreds of metres in width. Smaller single storey sediment bodies may also occur in association with the larger ones (e.g. the Snehvide Formation). The sediment bodies are composed of relatively coarse sandstones a n d / o r conglomerates (although finer-grained sandstones may be expected in the absence of coarse detritus). The channel deposits reflect fluctuating, though generally perennial discharge and may contain evidence of the development of braid bar complexes (e.g. the Snehvide Formation, the Chloritic Sandstone Formation). The second association present in the feeder zone is the "interchannel association", which is most commonly composed of mudstones/siltstones and relatively fine-grained sandstones deposited from overbank flows (e.g. the Chloritic Sandstone Formation). These sediments commonly exhibit signs of desiccation and may also reflect periodic ponding of water. Aeolian deposits may occur in association with, or alternative to the overbank sediments. In the examples presented in this paper, aeolian deposits are not reported from the feeder zone. However, Olsen and Larsen (1993b) describe an example from the East Greenland basin in which aeolian sand sheet and dune deposits dominate the interchannel association. The interchannel association usually occurs in sequences several metres thick. Distributary zone

In passing from the feeder to distributary zones a downstream decrease in the frequency and di-

369

mensions of channel bodies may be anticipated (Nichols, 1987). However, although the transition from the feeder zone to the distributary zone is marked by a distinct reduction in the mean grain size of the channel sediments, it is commonly associated with an increase in channel body density. This is thought to reflect the downstream splitting of major feeder channels, resulting in networks of distributary channels. In more distal reaches, multifurcation is likely to result in a complete spectrum of sandstone bodies ranging from those deposited by major rivers to minor single event, weakly channelized, flood sedimentation units. A simple two-fold subdivision of facies into coarse "in-channel" and fine "overbank" facies therefore seems inappropriate. If discharge across a system varies significantly with time we may expect to see large river channels reaching distal parts of the system during periods of high discharge and only small channels in the proximal parts during periods of low discharge (Nichols, 1987; Kelly, 1992, 1993). The higher preservation potential of larger channel bodies may thus blur any proximal-distal trend. In addition, bifurcation will produce a pattern of varying channel dimensions across the system and this could also result in the local preservation of small channels in proximal reaches and large trunk channels in distal parts of the system (Nichols, 1987). The width/depth ratio of distributary channels may increase (cf. Schumm, 1961; Nichols, 1987; Bull, 1991), or decrease (Mukerji, 1975) in a downstream direction. Experience from Devonian systems suggests that the former case is generally more common. The connectivity of sandbodies would also be expected to decrease down stream as channel frequency and depth decreases, although this may be compensated to some extent by bifurcation which increases the number of channels (Atkinson, 1986). Sanddominated sheetfloods will tend to produce laterally extensive sandstone units with generally good internal connectivity. The development of discrete lobes in the distal parts of the system may cause local increases in the density and connectivity between individual channel sandbodies (Nichols, 1987).

370

A general proximal-distal relationship is reflected in the density of distributary channel bodies and a tentative subdivision of the distributary zone into "proximal", "medial" and "distal" parts may be possible. "Proximal" parts are dominated by distributary channel deposits. In sand-prone systems the entire sequence may be composed of distributary channel deposits (e.g. the Snehvide Formation) reflecting braidplain characteristics. In depositional systems carrying larger amounts of suspended fines, interbedding of channel and sheetflood sandstones with mudstones may occur (e.g. the Sherkin Sandstone Formation). In the "medial" part, interbedding of channel sandstone bodies and sheetflood units occurs, with subordinate to subdominant flood basin mudstones (e.g. the Sherkin Sandstone Formation) or aeolian sandstones (e.g. the Rodebjerg Formation). The channel sandstone bodies generally exhibit limited thickness variations and may either reflect perennial discharge (e.g. the Rodebjerg Formation) or varying degrees of ephemerality (e.g. the Sherkin Sandstone Formation). Distributary channel sandstone bodies are generally sheetlike and commonly exceed 100 m in width (e.g. the R~debjerg Formation). In the "distal" part of the distributary zone channel density decreases. Distributary channel sandstone bodies may still be dominant (e.g. the R0debjerg Formation). More commonly, however, either flood basin fines (e.g. the Sherkin Sandstone Formation) or sheetflood sandstones dominate (cf. Tunbridge, 1984; the Snehvide Formation). Distributary channel sandstones occur both as single and multistorey bodies and generally do not exceed 100 m in width. Distributary channel deposits in general reflect ephemeral flow conditions. In fluvial systems carrying large amounts of suspended fines, flood basin mudstones may form a considerable percentage of the distal sequences (e.g. the Chloritic Sandstone Formation, the Sherkin Sandstone Formation). In systems poor in suspended load, aeolian sandstones may be common in distal sequences (e.g. the Rcdebjerg Formation).

s . B K E L I ~ / \ N D ]t f~LSI.N

Basinal zone

The basinal zone of terminal fans is probably governed by the suspended load/bedload ratios of the terminal fan systems and the character of adjacent depositional systems. High suspended load/bedload ratios resull in a dominance oi floodbasin mudstones and fine-grained sheetflood sandstones with rare ephemeral stream channel sandstones (Irish examples). Alternatively the basinal zone may be composed of playa mudflat fines (e.g. Tunbridge, 1984; Olsen. 1987). Low suspended load/bedload ratios may result in aeolian reworking of sands and a dominance of aeolian dune and sand sheet sandstones with subordinate flood basin mudstones, sheetflood sandstones and rare ephemeral stream channel sandstones (Greenland examples).

Summary (1) Terminal fans occur where sediment-laden streams decrease in size and vanish as a result of evaporation and transmission losses. They tend to form in arid or semi-arid regions which are characterized by a moisture deficit. (2) Distributary channel patterns are characteristic of terminal fans and reflect both toss of stream power and spatially/temporally fluctuating discharge. (3) Terminal fans are characterized by three basic zones; the feeder, distributary and basinal zones, each of which will have characteristic sedimentary deposits related to the subenvironment. (4) The size and character of terminal fan systems are closely related to the type of dis. charge/sediment supplied and the nature of contemporary depositional systems. High-discharge/ mixed-load systems tend to develop extensive fans and associated fine-grained floodbasins into which they prograde (e.g. Irish examples). Low to moderate discharge, sand-dominated systems tend to develop smaller fans that can interact in a complex manner with adjacent depositional systems (aeolian/fluvial) (e.g. Greenland examples).

TERMINAL FANS--A REVIEWWITH REFERENCETO DEVONIAN EXAMPLES

Acknowledgements S.B.K. acknowledges a BP research studentship for work in Ireland at University College Cork, and the continued support of the Reservoir Geology Division of the Geochem Group Limited. H.O. acknowledges a Carlsberg Foundation post doc. grant and additional technical support from the Geological Survey of Greenland in connection to his research in Greenland. The authors would like to thank Osman Abdullatif for providing unpublished information corncerning the Gash Fan, Sudan. Pat Meere and Shaun Sadler generously provided unpublished information on the Munster Basin. Shaun Sadler, John Collinson and Chris Fielding kindly suggested improvements to an earlier manuscript.

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