Taphonomy and depositional dynamics of Devonian shell-rich mudstones
Abstract Fossil preservation (e.g. articulation of multi-element skeletons) provides important insights into depositional dynamics. Detailed taphonomic analysis of marine mudstones and associated shell beds from the Middle Devonian of western New York indicates that these beds record primarily episodic sediment deposition of short duration. We propose a storm winnowing, mud blanketing model to explain the variety of shell and mudstone beds observed, and to explain apparent onshore-offshore gradients of shell bed characteristics. Shell beds themselves appear to have accumulated during relatively long spans of time ranging up to perhaps thousands of years. Most are internally complex and exhibit evidence for multiple events of burial and exhumation by winnowing. Shell accumulation facilitated colonization of the seafloor by hard substrate-encrusting organisms; these contributed to increasingly complex communities that were ultimately destroyed and permanently blanketed by thick mud layers. The sparsely fossiliferous mudstones, which compose up to 85% of the Hamilton stratigraphic section, mainly record very abrupt inputs of sediment; in some instances taphonomy of enclosed fossils indicates deposition of layers up to several centimeters thick (prior to compaction) within hours to a few days. In turn, some sequences of shell and mudstone beds were deposited within the life span of single colonial organisms, and thus also record only relatively short spans of time overall. This reinforces our conclusions that most time in mudstone sequences must be represented by discontinuities at sharp surfaces or thin condensed beds that bound more or less continuous bundles of shell and mudstone beds. Again, these often subtle discontinuities can be recognized on the basis of taphonomic criteria. Many complex shell beds as well as simple shell beds can be traced along depositional strike for tens of kilometers. Condensed shell beds only a few centimeters thick can commonly be correlated for 100 to 200 km along and across depositional strike. The correlatable nature of shell beds apparently reflects the temporal contrast between these beds and enclosing muds; on a regional scale, shell beds represent integrated accumulations that were deposited over spans of tens to thousands of years. Conversely, the mud layers that blanketed and terminated shell bed accumulations may represent very rapid depositional events (spanning hours to days), which are locally isochronous. Thus, the study of taphonomy of shell beds reveals a good deal of detail about their depositional history; these studies imply that the stratigraphic record is even more episodic and discotinuous than commonly recognized. Episodic deposition implies that while the stratigraphic record of epeiric seas may be adequate for examining details of community succession and paleontology, it probably can not generally be used for microevolutionary studies other than examination of patterns of stasis.