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From Lithostratigraphy to Sequence Stratigraphy: Still Chasing the Elusive Time Surface

Abstract

Geologic time resides at the heart of stratigraphy. Thus our attempt to understand, subdivide or formalize stratigraphic record is rooted in the Law of Superposition. Earlier in the practice of lithostratigraphy, rock units were expected to conform to time units, implying that lithostratigraphic boundaries follow time surfaces. Later, by demonstrating that lithostratigraphic units are diachronous in nature, the concept of time-stratigraphy (i.e., chronostratigraphy) was introduced as a better way to subdivide the stratigraphic record. Since then, numerous suggestions were made on how to practice chronostratigraphy (e.g., sequence stratigraphy) using both conformable and unconformable stratal surfaces, which are generally considered time surfaces (i.e., isochronous). However, expression of time in the stratigraphic record is far more complex than we realized hitherto. The idea that physical surfaces can serve as chronostratigraphic boundaries breaks down upon closer scrutiny, particularly for dynamic depositional systems. 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. These are particularly true for depositional systems where undulating geomorphic units (e.g., bedform, bar and channel) 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 deposits below the same surface. The above observations have critical implications in testing sequence stratigraphic concepts. For example, various composite stratal surfaces including unconformities, which are not time barrier as argued above, are being used widely as sequence stratigraphic surfaces. Recently, the concepts and/or definitions of incised valley fills and subaerial unconformity have been challenged. Notably, this issue can also be linked with the debate of whether to place the correlative conformity at the beginning or end of a sea level fall. As technology advances, the future of (sequence) stratigraphy lies in understanding the rock record in a proper fabric of 4D spacetime.