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Abstract

The vastness of time is largely beyond human observation, but how aware are most geologists of the concept of time? Time spans of just a few thousands of years may become unfamiliar when moving from the modern, observable, and quantifiable, sedimentary processes acting on decadal to centennial time scales to the intricate series of depositional events discontinuously preserved in the rock record. Our experiential concept of geologic time built on the sequence stratigraphy of chronologically well-constrained, late Quaternary successions delineates a virtually unexplored hierarchy of hiatal surfaces (and condensed intervals) on 102 to 105-year time scales; i.e., below the chronologic resolution of most dating techniques commonly used to interpret the ancient stratigraphic record.

In continental-margin settings, the fourth-order, Late Pleistocene-Holocene depositional sequence is punctuated by sedimentary hiatuses, and highly episodic deposition appears to be the rule rather than the exception. Systems-tract and parasequence boundaries record long periods of non-deposition, erosion, and/or stratigraphic condensation, and as little as 20% of elapsed time is represented by preserved lithofacies assemblages. In the Po River basin, a significant stratigraphic break having a cumulative duration of up to 80 ky has been produced by the prolonged, stepped phase of eustatic fall and subsequent lowstand between about 120 ky and 20 ky BP. Fluvial channel-belt sand bodies developed during relatively short time periods (~10–15 ky). Early Holocene isolated transgressive sand bodies extend for tens of kilometers along dip, spanning intervals of time of just a few centuries. In coastal-plain successions in this system, up to 50% of geologic time is in the interval just below parasequence boundaries, during formation of relatively condensed peat-bearing intervals. Finally, progradational sets of highstand deltaic parasequences, up to 30-m thick, can make up to 95% of the total volume of Holocene deposits, but embrace just 10% of elapsed time.

Intervals of older fluvial and shallow-marine strata having sizes and architectures similar to the Po River system, which are chronologically constrained at much lower resolutions, tend to be interpreted to have developed on larger temporal scales. In these cases, severe distortions can be generated by the over-generalized assumption that sediment packages between regional unconformities represent relatively continuous successions of strata. This assumption will result in a bias towards estimates of sedimentation rates, event frequencies or durations, and sediment fluxes that can be incorrect by orders of magnitude. In addition, stratal successions bounded by surfaces of chronostratigraphic significance may not be as closely genetically related as commonly supposed. Appreciating the highly fragmented nature of the sedimentary record can fundamentally change the interpretation of hierarchical stacking of parasequences and the time scales of formation of ancient alluvial and deltaic depositional systems. We illustrate the impact of this appreciation by comparing the late Quaternary Po Plain basin stratigraphy against older strata of the Eocene Escanilla Formation and Cretaceous Blackhawk Formation strata.

It is generally accepted that geologic time can be largely unrepresented by rocks, although this concept has been poorly clarified and only roughly estimated. In this paper, we extend the uniformitarian principle that “the present is the key to the past” to encompass a broader vision in which, at least for certain periods in the Earth’s history, “the recent past is the key to the deep past.”

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