Dott 1996

SEDIMENTARY GEOLOGY ELSFiVIER

Sedimentary

Geology

104 ( 1996) 243-247

Episodic event deposits versus stratigraphic sequencesshall the twain never meet? R.H. Dott Jr. Department

of Geology and

Geophysics, University of Wisconsin, Madison, WI 53706.

Accepted 27 September

USA

1995

Abstract In the early half of the 20th century, questions were raised about different rates of deposition reflected in the stratigraphic record and of small gaps or diastems. Even more discussion centered upon rhythms and cycles of deposition-at one extreme were glacial varves and at the other were Carboniferous cyclothems. After World War II, interest in stratigraphic cycles declined. Then the turbidity current revolution stimulated interest in event deposits, which interest has surged again recently with a focus upon storm deposits. Meanwhile, the recent dramatic growth of sequence stratigraphy has rekindled interest in both cyclicity and eustasy. The two themes-events and cycles-should be better integrated, for there is considerable confusion about the interpretation of high-frequency sequences. There is also a need to reconcile the current fad for Milankovitch-related sedimentary cycles versus more or less random event deposits. The most familiar event deposits are turbidites in deep-water and tempestites in shallow-water environments. More subtle are the diastems, which include non-depositional surfaces as well as scoured surfaces. Other processes that can produce event deposits include avalanches and tsunamis. Potentially, any type of event deposit could be mistaken for a sequence boundary. For example, submarine megabreccias could be formed either by a seismic event unrelated to any sea level change, or by slope failure resulting from a eustatic fall associated with a sequence boundary. To surmount the intellectual barrier to alternate interpretations requires careful attention to processes, time resolution, and objective tests for periodicity.

1. Introduction I .I. Episodic deposition

Episodic event deposits have received much attention during the past 15 years. For example, Ager eloquently characterized the stratigraphic record as “a lot of holes tied together with sediment” (1980, p. 3.5). Sadler (1981) and Schindel (1982) presented elegant completeness tests based upon apparent accumulation rates. Van Andel (1981) speculated about 0037-0738/96/$15.00 SSDf 0037-0738(95)00

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1996 Elsevier Science I3 I -X

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the implications of these completeness tests and the book Cyclic and Event Stmti$cation (Einsele and Seilacher, 1982) emphasized the importance of punctuated deposition using diverse examples from the stratigraphic record. I have also argued that much of the sedimentary record reflects discontinuous or episodic events, such as storms, more than continuous, day-to-day processes (Dott, 1983, 1988). Aigner and Reineck (1982) and Aigner (1985) documented the importance of storm deposits in both modem and ancient strata. More recently, Einsele et al. (1991)

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and Ager (1993) have produced second books stressing the punctuated perspective of the stratigraphic record. And Sadler (1993) has argued that apparent average rates of unsteady processes decrease as longer time spans are studied, which has important implications for accommodation space and thus sequence stratigraphy. The view that the stratigraphic record reflects processes with greatly varying rates and frequencies of recurrence is not new. Early in the century, Barrell (1917) recognized a disparity between modem depositional rates and ancient apparent accumulation rates, which challenged a subtle implication of strict uniformitarian thinking that most sedimentary rocks reflect continuous, day-to-day processes. He proposed, instead, that the ancient record must contain many subtle breaks (Ager’s holes) with time values of the order of years to decades; he coined diastem for such intraformational breaks. Although Barrel1 was commonly cited, little further attention was given to the issues he raised until the 195Os, when both turbidity currents and the hundred-year flood concept became well established. Then the question of whether rare, large-magnitude or frequent, smallermagnitude processes leave a greater impact upon the sedimentary record became a matter of lively debate. Simpson (1952) and Gretener (1967) presented elegant commentaries on the probability of rare events in a geological context. The more recent stratigraphic completeness analyses (e.g. Sadler, 1981; Schindel, 1982) built upon such stochastic arguments. More recently, a symposium on ‘convulsive events’ (Clifton, 1988) reinforced the concept of episodic deposition.

Geology 104 (1996) 243-247

(Vail et al., 1977). Although Sloss has always attributed the primary cause for sequences to tectonism (e.g. Sloss, 1991), Vail and his associates at Exxon Production Research Co. have always considered eustasy to be the primary factor. Evidence from the deep-sea record that Quatemary glacial-interglacial events correlate well with the Milankovitch orbital cycles gave strong impetus to a paradigm of periodicity for many older stratigraphic patterns as well (e.g. Fischer, 1981). The notion of globally synchronous rhythms dates back at least to the turn of the century and the enormous influence of T.C. Chamberlin’s hypothesis of diastrophic control of stratigraphy with its corollary of periodic worldwide unconformities (Dott, 1992a). Between 1900 and 1950, rhythmic cycles of all sorts were popular. Besides pulsations of the sea a la Chamberlin, there were annual glacial lake varves, climatic cycles, Carboniferous cyclothems, global tectonic cycles, and evolutionary rhythms (Dott, 1992b). Periodic cycles were being discovered (or invented) everywhere and books appeared with such eloquent titles as Pulsation Theory (Grabau, 1940) and Pulse of the Earth (Umbgrove, 1947). By 1950, however, enthusiasm for periodic oscillations had been largely damped out (e.g. Gilluly, 1949; Henbest, 1952; Moore, 1959). After nearly three decades in eclipse, however, the concept of global rhythms has returned to popularity again thanks to a large body of evidence of global, sequence-bounding unconformities obtained by the petroleum industry with highresolution seismic profiles plus evidence from the deep-sea record, which supports the importance of the Milankovitch cycles for climate (Dott, 1992b).

1.2. Periodic sequences Meanwhile, another thread of stratigraphic thinking has been spun out during the past thirty years. Large-scale discontinuous stratigraphic patterns were being recognized during the 1950s when Sloss first formally defined six widespread, unconformitybounded sequences across the North American craton @loss et al., 1949; Sloss, 1963). While these original sequences (now sometimes termed supersequences) had time spans of the order of 10’ years, later refinement emphasizes higher-frequency packages with time spans of the order of 105-IO6 years

2. Can event stratigraphy and sequence stratigraphy co-exist? Presently, sequence stratigraphy controlled by periodic eustatic fluctuations seems to be the dominant paradigm in sedimentary geology. Conversely, more or less random episodic processes, which received so much attention in the 1980s are being eclipsed. This trend seems ill founded, however, and is bound to produce serious errors of interpretation if left unchallenged.

R.H. Dott Jr./ Sedimentary Geology 104 (1996) 243-247

2.1. Event stratigraphy

Event deposits do exist and in large volumes in many sedimentary basins. Consider, for example, turbidites, which perhaps represent our finest example of episodic deposition. Most of the sediment in a succession of such deposits accumulated during only a fraction of the total time represented by that succession. Suppose, for example, that 100 sandstone turbidite beds averaging 20 cm thick alternate with pelagic shale beds averaging 1 cm thick, all of which accumulated during 100,000 years. Then 99% of the total thickness would have been deposited by instantaneous currents that occurred only once per 1000 years on the average. Tsunamis (Coleman, 1968) and major storms produce similar sedimentary records, which are difficult to distinguish in ancient strata, but storms have received much more attention. Consider the real world of hurricanes in the Gulf of Mexico. The statistics of hurricane frequency suggest that any portion of the northern coastline of the Gulf of Mexico should be crossed by an average of one storm per century (Tannehill, 1969). Although rare from the average human’s perspective, phenomena like Gulf Coast hurricanes, the 500-year flood of the upper Mississippi River in 1993, and tsunamis, as well, must be regarded as common in a geological context. Nonetheless, all of these episodic processes are very different from fairweather, day-to-day ones, but whether such rare, energetic or the common, mild process leaves the greater imprint upon the sedimentary record is not always clear. I have suggested that we should distinguish between positive and negative deviations from the norm for any given environment. Thus storms, volcanic eruptions, landslides, tsunamis, and asteroid impacts are much greater than the norm in intensity both of energy and of sediment accumulation. Conversely, the slowing or cessation of deposition and stunting or annihilation of organisms obviously fall below normal intensities of energy and accumulation. Positive deviations tend to produce either erosion surfaces due to sediment removal (e.g. Barrell’s diastems) and/or abnormal volumes or coarseness of deposits; chaotic textures, as in landslides, may also result. Negative deviations tend to produce condensed sections, hardground and firmground sur-

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faces, and possibly skeletal depauperate or death assemblages. Besides physical events, which are the main subject of this symposium, purely chemical events also may produce important episodic deposits. For example, an oxygen crisis might be reflected by a widespread fossil death assemblage, whereas a brief salinity crisis could be reflected by a skeletal death assemblage associated with a basinwide evaporite stratum. The preservation potential of episodic event deposits must be considered, for if such deposits are commonly removed or obscured after deposition, then their importance may seem of little more than theoretical interest. In some very dynamic environments, such as a volcanically active region, repetition of high-energy episodic events may remove the record of most earlier events, thus distorting our impression of the frequency and total importance of such events. Bioturbation also has great potential for obscuring the record, as is evidenced by the almost complete obliteration by animals of the famous 1961 Hurricane Carla storm sand layer in the northern Gulf of Mexico (McGowan, oral communication, 1981). Diagenetic changes after burial also have the potential to obscure the records of event deposition. In summary, there is a danger of underestimating the true importance of event deposition because of postdepositional modification or removal of some of its record. Very careful field examination is necessary to minimize this danger. 2.2. Sequence stratigraphy Valid stratigraphic sequences certainly do exist, as well, and are especially clear at the larger scale of supersequences and sequences. It is at the scale of parasequences that conflicts among genetic interpretation may arise. Without going into detail, consider the following five scenarios for potential misinterpretation. (1) Scoured surface (diastem) and/or an abrupt textural change. Episodic interpretation: a random storm or tsunami event. Sequence interpretation: parasequence boundary with eustatic fall. (2) Thin intraclast conglomerate layer. Episodic interpretation: random storm or tsunami event. Sequence interpretation: flooding surface with eustatic rise.

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R.H. Dott Jr./ Sedimentary Geology 104 (1996) 243-247

(3) Carbonate-clast megabreccia stratum. Episodic interpretation: platform failure by earthquake or storm. Sequence interpretation: lowstand deposit due to eustatic fall. (4) Hummocky interval within a coarsening-upward succession. Episodic interpretation: storm deposition below fairweather wave base within a prograding package during a sea level stillstand. Sequence interpretation: storm deposition in a transgressive systems tract. (5) Thin marine shelly sandstone abruptly overlying eolian sun&tone. Episodic interpretation: coastal flooding by storm surge or tsunami. Sequence interpretation: transgressive flooding by eustatic rise. Periodicity tends to be a tacit assumption of most sequence stratigraphy interpretations, and one or more of the Milankovitch cycles is soon invoked as the driving control for the inferred sequences. Periodicity, however, is not an inevitable logical corollary of sequences. Although there are good reasons for expecting some periodic phenomena to be recorded in the rock record, it is inescapable that geologists have an excessive passion for periodic cycles. This tendency undoubtedly follows from the universal experience of natural rhythms, such as diurnal, tidal and seasonal cycles. It also may reflect millennia of influence of the Aristotelian Greek view that all of nature is cyclic, but it mostly reflects the human passion for order and simplicity. Moreover, because periodicity provides both order and simplicity, it also resonates nicely with time-honored uniformitarianism. The danger is that such subconscious predilections may so deceive us that we invent periodic order for which there really is no hard evidence.

3. Conclusions Although episodic and periodic eustatic explanations of sedimentary phenomena have both received much attention during the past 15 years, there is a growing tendency to interpret most stratigraphic patterns in sequence terms with little consideration of alternatives. Because periodicity and eustasy are so susceptible to circular reasoning, this is a dangerous trend. It should be incumbent upon all to test any hypothesis of periodic eustatic changes as rigorously as possible (with ever-finer dating, Markov analysis,

spectral analysis, etc.) and to consider other explanations, such as random or episodic causes like storms or tsunamis.

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