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Mismatch Between Time Surface and Stratal Surface in Stratigraphy
In stratigraphy, stratal surfaces are generally considered time surfaces (i.e., isochronous). However, expression of time in the stratigraphic record is far more complex than we realized hitherto. Here I show that stratal surfaces are commonly composite in nature, and that, from ripple-scale (∼ 10–6 year) to at least sequence-scale (∼ 106 year), time surfaces cross these stratal surfaces. Moreover, in addition to elapsing vertically, time elapses far more (> 90%) laterally along composite surfaces. These are particularly true for active depositional systems where undulating geomorphic units (e.g., bedform, bar, channel, and clinoform) migrate in time and space to fill a basin. Migration of geomorphic surfaces (i.e., time surfaces) is commonly associated with synchronous sediment deposition, erosion, and bypass in 3D space, prompting preservation of highly shredded time surfaces and generating composite stratal surfaces at each level of hierarchy from ripple- to basin-scale “clinoformal” packages. Deposits above such a stratal surface are not everywhere younger than the deposits below the same surface. This partially violates the Law of Superposition, which has been overextended to state that each layer is always and everywhere younger than the layer beneath it. In reality, this Law applies only in one location moving strictly vertically through a rock succession as in a well. These observations have far-reaching implications in interpreting the earth history, which depends enormously on our proper understanding of how time is expressed in the vast sedimentary record.
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