A LINKAGE BETWEEN STRATIGRAPHIC HIERARCHY, SEDIMENT ACCOMMODATION, AND PROCESS TIME SCALES: A FLUVIAL VIEW

Stratigraphic units are well reported to record patterns that are routinely attributed to driving depositional processes. Though commonly appearing stochastic in direct observation, modern systems still record depositional patterns. The linkage of process to stratigraphic expression is based primarily upon the observation that the patterns modern systems produce replicate the patterns observed from the ancient. Uniformitarianism implies that this commonality in pattern translates to a commonality in causal mechanism. Rates of ancient sediment accumulation and rates of modern stratal deposition often appear inconsistent in that the rates of net vertical accumulation in most ancient systems are too slow to accommodate the thicknesses accumulated by comparable modern processes. Part of this inconsistency can be explained by two factors. First, longer-term process will ultimately record as shifts in the many successive “snap shots” preserving the shorter-term processes. Processes are thus best recorded by the elements in the part of the stratigraphic hierarchy which deposit over the time scale most characteristic to the time scale of the process. For instance, processes that operate over a few millennia are best preserved by stratigraphic units that form over millennia (e.g., channel belts, terrace units, etc.), whereas processes that occur over decades are poorly recorded by these units and are more likely recorded in shifts in elements that form over these time periods (e.g., channel scours, bar forms, etc.). Second, much of the volume of sediment deposited is accommodated laterally instead of vertically, providing the unaccounted space required to store apparently thicker units in apparently thinner vertical space. Errors in interpretation occur when processes and depositional products are not well matched in their stratal hierarchy, and/or the capacity for lateral storage of narrow sedimentary units is not accounted.