The Present as the Key to the Past, and Vice Versa

We have all heard the cliche“the present is the key to the past.” It was one of the great generalizations to emerge from James Hutton’s enunciation of the principle of uniformitar- ianism toward the end of the eighteenth century. Charles Lyell’s work a half century later seemed to nail it down forever as a cornerstone of the still unborn science of sedi- mentology. Yet it is only true in a limited sense. Undoubt- edly the study of modern depositional environments provides the essential basis for modern facies studies, but there are at least a dozen major problems that emerge to confuse this work. It turns out that the past is also a very important key to the present, that many geomorphological processes can best be understood by adopting a geological perspective and looking at the ancient record. The art of facies modeling is therefore a two-way process.

The greatest advantage in studying modern environments is that we can observe and measure sedimentary processes in action. We can measure current strength in the rivers and oceans; we can observe at least the smaller bedforms moving and evolving; we can measure temperatures and salinities and study the physics and chemistry of carbonate sedimen- tation and the critical effects of organic activity in the photic zone (shallow depth zone in the sea affected by light pene- tration). We can sample evaporite brines in inland lakes and deep oceans, such as the Red Sea, and deduce the processes of concentration and precipitation. We can put down shallow core holes and study the evolution of the sedimentary environment through the Recent to the present day. Many physical, chemical, thermodynamic, and hydrodynamic sedimentary models have resulted from this type of work since the 1960s. As noted in the previous section, for some sedimentary environments, work of this type has constituted the major advance in our understanding. Examples of early facies models that depended largely on studies of modern environments include those for deltas (Coleman and Wright

1975; Galloway1975), barrier islands (Bernard et al.1959; Dickinson et al. 1972), tidal inlets (Kumar and Sanders

1974), clastic tidal flats (Evans 1965; van Straaten 1951,

1954; Reineck and Singh1980), tidal deltas (Hayes 1976), and sabkhas (Shearman1966; Kinsman1966,1969).

Some of the most difficult problems to resolve when using actualistic models stem from the inadequate time scale available to us for observation purposes. A very persistent geologist may be stubborn enough to pursue the samefield project for 10, perhaps even 20, years. Aerial photographs might push observations of surface form as far back as about 1920 at the earliest. Old maps may go back for another 100 years or more, but become increasingly unreliable. Weather records may be available back into the nineteenth century; stream gauge data have been collected for only a few decades. It is difficult to assess the relevance of a 100-year record to a geological unit that may have taken a million or more years to accumulate. Have the last 100 or so years been typical? Were the same sorts of processes occurring at the same rates in the distant geological past? As discussed in the following paragraphs the answer is a qual- ified maybe.

The most important aspects of this question are the dif- ficulty of assessing the importance of ephemeral events and judging the preservation potential of deposits we can see forming at the present day. Dott (1983) and Reading and Levell (1996) have called attention to the need to distinguish between normal and catastrophic sedimentation. Normal processes persist for the greater proportion of time. Net sedimentation is usually slow. It may be nil or even negative if erosion predominates. Normal processes include pelagic settling, organic growth, diagenesis, tidal andfluvial currents (Reading and Levell 1996). Reading and Levell (1996)

distinguished catastrophic processes as those that “occur almost instantaneously. They frequently involve ‘energy’ levels several orders of magnitude greater than those oper- ating during normal sedimentation.” They may deposit a small proportion of the total rock and give rise to only an occasional bed, or they may deposit a large proportion of the total rock and so become the dominant process of deposi- tion. Examples of catastrophic processes includeflash floods in rivers, hurricanes, and sediment gravityflows. Although geologists have studied many modern flash flood deposits and the effects of several recent hurricanes (e.g., Hayes

1967), we cannot be sure that their magnitude and frequency at the present day are the same as that in some past period of interest without attempting to obtain some geological per- spective from studying the ancient record. The most violent and geologically important event may be one that only occurs every 200 or 500 years and has not yet been seen. Sediment gravityflows are thought to be the chief agent of erosion of submarine canyons, but in spite of years of oceanographic observation, no major flows have actually been observed there (Shepard et al.1979).

Ager (1973, 1981) and Dott (1983) argued that many stratigraphic sequences contain more gaps than record, and that significant proportions of the sedimentary record are deposited in a very short time by particularly violent dynamic events. Such events (hurricanes, sediment gravityflows, flash floods, etc.) are rare and difficult to study in action. Most of our energy as sedimentologists is expended in studying the less violent processes that occupy most of geological time but may contribute volumetrically far less sediment to the total record. Ager (1986) re-examined a well-known basal con- glomerate in the Jurassic Lias of Wales, and concluded that it was a storm deposit formed very rapidly. In fact, he face- tiously suggested that (p. 35)“it all happened one Tuesday afternoon,” a remark that was deliberately designed to draw attention to the difference between “normal” and “event” sedimentation (we return to this topic in Chap.8).

Studies of modern environments also suffer from the fact that many of the deposits we see forming at the present day are lost to erosion and never preserved. Thus, the geological record may be biased. The bias may be in favor of more deeply buried sediments, which the geologist, scratching the surface with trenches and box cores, never sees. For exam- ple, Picard and High (1973) published a detailed study of the sedimentary structures of modern ephemeral streams, but many of the structures are sufficiently ephemeral to be rarely, if ever, found in the ancient record. Fluvial flash flood deposits commonly are preferentially preserved because they infill deep scours below the normal level of fluvial erosion (Miall1996, Chap. 8). Many of ourfluvial facies models are based on studies of modern rivers in upland regions under- going net degradation. How relevant are they to research in some of the great ancient alluvial basin that by the thickness

of preserved deposits demonstrate a long history of aggra- dation? Facies models for barrier and shoreface environ- ments are subject to similar constraints. For years, the classic Galveston Island (Texas) model of coarsening-upward beach accretion cycles dominated geological thinking (Bernard et al.1959), but more recently it was realized that the barrier sediments may be removed by lateral migration of deep tidal inlets and that many barriers consist of superimposed inlet deposits with a superficial skin of wave-formed shoreline sediments (Kumar and Sanders1974). Hunter et al. (1979) argued that many shallow subtidal deposits are systemati- cally removed by rip currents and are never preserved in the geological record.

Some of the best studies of modern environments are those that use shallow drill cores to penetrate into pre-Recent deposits. Such sediments can be said to have“made it” into the geological record, and yet they can be placed in the context of a still extant and presumably little modified modern environment. Some work on modern turbidites (Bennetts and Pilkey1976), anastomosed rivers (Smith and Smith1980), and reefs (Adey1975; Adey et al.1977; Shinn et al.1979) is of this type.

In two important ways, the present is quite unlike the past, and is therefore an unreliable laboratory for recon- structing sedimentary environments. The Pleistocene ice age generated several geologically rapid changes of sea-level culminating in a major rise and transgression since about 12,000 years ago. Second, the present configuration of continents and oceans is a unique pattern, different from any in the past because of the long history of sea-floor spreading, rifting, subduction, and suturing. This plate movement has had an important effect on some of the broader aspects of facies models.

Because of the recent sea-level rise, modern continental shelf, shallow-marine, and coastal plain deposits around the world have been formed under transgressive conditions. Sea level was approximately 150 m lower during the Plio-Pleistocene glacial phases, rivers graded their profile to mouths located near the edge of present continental shelves, and carbonate platforms, such as the Bahamas, were exposed to subaerial erosion and may have developed extensive karst systems. Submarine canyons were deeply entrenched by active subaerial and submarine erosion (Shepard 1981; McGregor 1981). During the rapid transgressions that fol- lowed in interglacial phases, shoreline sands were continu- ally reworked. On the Atlantic shelf off North America, extensive barrier islands were formed and receded into their present position (Swift1975a). River valleys were drowned and filled with estuarine and deltaic deposits. Submarine canyons were commonly deprived of their abundant supply of river-borne detritus, as this was now deposited on the landward side of a widening and deepening continental shelf. This had a drastic effect on the rate of growth of some

submarine fans. Carbonate sedimentation began afresh in warm, detritus-free waters, over resubmerged platforms, but the local water depths, circulation patterns, and facies dis- tribution may have been partly controlled by erosional topography (Purdy1974; although this is disputed by Adey

1978). We therefore have excellent modern analogues for studies of rapid transgressions in the geological record, but few for rapid regressions or for periods of still stand. The Mississippi and other large deltas are good examples of regressive deposits built since the last ice age, but they represent an environment that may be characterized by unusually rapid progradation. Some have maintained that most modern continental shelves are covered by relict sed- iments, implying that their study may not be of much geo- logical relevance (Emery 1968), but more recent work has shown that the dynamic effects of tidal currents and storms do in fact result in continual change (Swift et al.1971).

Miall (2014a,2015) has argued that basing stratigraphic interpretations on comparisons with the post-glacial record is a mis-application of the principal of uniformitarianism for two important reasons: (1) the issue of preservation—the present post-glacial record exemplifies the unfinished nature of the geological preservation machine. A future glacioeu- static fall in sea level would remove much of the sedimen- tary record along continental margins and within estuaries, and subsequent (future) cycles of rise and fall would gen- erate a long-term geological record by the superimposition of the lowermost fragments of successive cycle. (2) Rates of processes calculated from study of the post-glacial record (e.g.,fluvial and deltaic channel avulsion; lobe switching on deltas and submarine fans) are orders of magnitude greater that the rates that can be calculated from apparently similar processes in the geological past, suggesting that uniformi- tarianist comparisons of processes may be incorrect. For example, Miall (2014a) suggested that changes in fluvial channel stacking patterns in the ancient record are not reflections of changing rates of sedimentary accommodation but record proximal-distal shifts influvial styles in response to allogenic forcing.

Rapid transgressions may have occurred commonly dur- ing other ice ages in the geological past (Miall 2010), but most other changes in the geological past were somewhat slower. For two lengthy periods in the Phanerozoic, our suite of modern analogues is particularly inadequate. These were the times of high global sea-level stand during the Ordovician-Silurian and again in the Cretaceous, when vast areas of the world’s continents were covered by shelf seas. We simply do not have modern equivalents of these huge inland seas, and many of the large shelf seas that do exist, such as the Bering Sea and Yellow Sea, have yet to be studied in detail. Likewise, there are no modern analogues for the very large evaporite basins that exist in the geological record (James et al.2010).

Ginsburg (in Byers and Dott1981) discussed the problem of interpreting Cambro-Ordovician carbonate banks of the North American craton. These are up to hundreds of kilo- meters long and thousands of kilometers wide. “The dilemma is how the vast extent of the banks, most of which suggest carbonate production and deposition in but a few meters or less of water, could all be bathed in normal marine or slightly restricted water. Would not such banks form major circulation barriers?” (Ginsburg, as reported by L.C. Pray, in Byers and Dott1981). It has been suggested that the banks are actually diachronous, and developed by seaward progradation, or they may have been crossed by“irregular to channelized deeper water areas facilitating water circula- tion.” Tidal currents certainly would have been able to assist with the latter. Careful biostratigraphic work has now demonstrated the diachronous nature of these tidal deposits. Runkel et al. (2007) showed that the Sauk sequence in Wisconsin consists of suites of superimposed, extremely low-angle, clinoform sets.

Climatic patterns and the network of oceanic currents are controlled by global plate configurations. In many respects, our present geography is unique. Therefore, we have modern analogues for situations that may not have existed in the past and, conversely, we cannot replicate certain conditions that did exist. For example, the Mesozoic Tethyan Ocean and the Pangea supercontinent had profound effects on climate and water circulation and hence on sedimentation patterns, and we have only generalized models for interpreting them. The study of paleoclimates is beyond the scope of this book, but is considered in an excellent review by Frakes et al. (1992). The last group of problems with the actualistic modeling method arises from the important effects organisms have on sedimentary processes. Plants and animals have, of course, both evolved profoundly since Archean time. Therefore, in many environments, sedimentation is controlled or modified by sediment-organism relations that did not exist, or were different, in the geological past (James et al. 2010). Many authors, beginning with Schumm (1968), have speculated on the implications of the evolution of land plants on fluvial patterns. Vegetation has a crucial effect on stabilizing channel banks, colonizing bars and islands, as controlling chemical weathering, sediment yield, and discharge fluctu- ations. Our typical braided and meandering fluvial facies models reflect these effects as they have been studied in temperate, humid, and hot and cold arid climates. Going back in time, the evolution of grasses in Mesozoic time must have changed geomorphic patterns profoundly. Abundant land vegetation is thought to have first appeared in the Devonian, and prior to that time the majority of river channels may have been unconfined, ephemeral, and brai- ded, as in modern arid regions (Davies and Gibling2010a,

b). Modern deserts are commonly used as analogues for pre-Devonian rivers, but there is no reason why they had to

be arid. However, we have no modern analogue for a humid, vegetation-free environment with which to study the pre-Devonian except, possibly, the south coast of Iceland.

Turning to shallow marine environments, the same kinds of difficulties apply. H.E. Clifton (in Byers and Dott1981) pointed out that salt marsh vegetation, so important on modern tidal flats, did not exist prior to the Cenozoic. Similarly, foraminifera, which provide sensitive bathymetric indicators for younger sediments, particularly those of Cenozoic age, did not exist in the Paleozoic. James et al. (2010) discussed the implications of these various evolu- tionary developments on changes in carbonate depositional environments.

The ecology of forms that are now extinct may be difficult to interpret, which makes them less useful in facies studies (Sect.3.5.8). Instead of providing independent, unambigu- ous environmental information, as do still living forms, it may be necessary to interpret them with reference to their sedimentary context, which itself may be of uncertain origin. Functional morphology is studied to determine probable habits, but many uncertainties may remain. In the Precam- brian, most sedimentary units are entirely devoid of fossils, and here problems of lithofacies interpretation without sup- porting fossil evidence may become acute. Many Precam- brian units have been reinterpreted several times for this reason, as different environmental criteria are brought to bear on particular problems. Long (1978) discussed many of these difficulties with reference to the recognition of fluvial deposits in the Proterozoic.

Reviews of Precambrian clastic sedimentation systems by Eriksson et al. (1998,2013) documented several major dif- ferences in the preserved Precambrian record relative to that of the Phanerozoic. Marine shelf deposits are characterized by very uniform suites of sediments lacking distinctive ver- tical trends. Fluvial deposits are dominated by those of braided style. Foreshore deposits are rare, and eolian dune deposits appear to be absent in rocks older than about 1.8 Ga.