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Master of Science (MS)
Rice, Melissa S.
Clark, Douglas H., 1961-
Sedimentary strata display a range of repetitive patterns from interbedded lithofacies through recurring sequence stratigraphic systems tracts. Highly structured, large-scale patterns are commonly ascribed to cyclic allogenic forcings such as eustasy and climate. In contrast, autogenic processes are typically thought to impart stochastic noise or limited small-scale structure on stratigraphy. Recent studies indicate some autogenic processes in fluvial and fluvio-deltaic systems such as the large-scale compensational deposition (i.e., the tendency for a channel to occupy and fill topographic lows in a basin), can occur on spatiotemporal scales that may overlap with some allogenic forcings. These autogenic processes could impart deterministic, highly structured patterns in stratigraphy. Thus, assuming autogenic deposition is stochastic may lead to overinterpreting stratigraphic organization as externally driven. Herein I evaluate this organization using two case studies.
Beds are the fundamental unit from which stratigraphic patterns are built, and here I evaluate bed thickness patterns within a purely autogenic experimental depositional system as well as in a field-scale basin potentially influenced by precession-scale orbital forcings (early Paleogene Willwood Formation, Bighorn Basin, Wyoming, USA). The purely autogenic experimental system (TDB-10-1) was created in the Sediment Dynamics Laboratory at Tulane University. In 67% of these synthetic stratigraphic sections, the spectral analysis identified statistically significant repetitions in bed thickness (at 99% confidence interval). However, the period of these cycles was not uniform, and in only 4% of cases was the identified period consistent with the known compensation timescale (222 minutes). The early Paleogene Willwood Formation data was recovered by the Bighorn Basin Coring Project in 2011. In the field study area, bed thicknesses with linearly interpolated ages from three cores were submitted to the same spectral analysis wherein cycles with periods of 3.5 kyrs, 3.7 kyrs and 15.0 kyrs were identified. These do not correspond to precession-scale (21 kyrs) variability previously identified by other researchers. The field and experimental results suggest noisy autogenic processes can produce spurious sub-Milankovitch cycles in stratigraphy, and that long-term autogenic compensational behavior is not cyclical. Moreover, experimental and field cases illustrate that it is possible to produce repetitive stratigraphy that does not predictably occur across the basin. These and other stratigraphic repetitions have been coined autocyclic in previous studies. I suggest the community restrict the widely adopted term “cyclic” to those processes with regularity in the time domain across basin, and adopt “auto-repetitions” for local autogenic processes with irregularity in the time domain.
alluvial, autogenic, allogenic, autocyclicity, allocyclicity, fluvial, quantitative, precession, compensation timescale, avulsion, redfit, bed thickness distribution, bed thickness
Western Washington University
Subject – LCSH
Beds (Stratigraphy)--Wyoming; Milankovitch cycles; Geology--Periodicity; Alluvium--Wyoming; Lithofacies--Wyoming
Bighorn Basin (Mont. and Wyo.)
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Phillips, Kristopher D., "Cycles or Repetitions: A Quantitative Analysis of Alluvial Bed Thicknesses" (2022). WWU Graduate School Collection. 1112.
Available for download on Saturday, May 20, 2023