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Controls on the Architecture of Lower Ismay Zone of the Honaker Trail Formation (Pennsylvanian), San Juan River, Utah, USA

Lechtenberg, Karen Suzanne
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Abstract
This study of the Pennsylvanian Lower Ismay zone of the Paradox Formation, Paradox Basin, Utah, USA intends to improve understanding of build-and-fill processes and carbonate sequence stratigraphy. Closely spaced, centimeter-scale stratigraphic sections reveal lateral and vertical heterogeneities in phylloid algal bafflestone and packstone of a mound-building phase and in fossil-rich wacke-packstone of a topography-filling phase. Ten lithofacies and one sublithofacies were documented through field study and petrographic analyses. Facies are organized into 10 units within 2 sequences distinguished by lateral geometries and surfaces representing changes in depositional environment. Sequence 1 comprises Units 1-5 and provides evidence of an overall relative shallowing from 50-100m depositional depth to subaerial exposure. A relative sea-level rise was recorded between Units 4 and 5. Sequence 1 consists of lithofacies 1 through 5: 1) Black Laminated Mudstone (BLM); 2) Spicule Mudstone (SM); 3) Crinoid Packstone (CP); 4) Algal Bafflestone (AB); and 5) Algal Packstone (AP). The algal facies (Lithofacies 4 and 5) created relief-building geometries. The sequence was exposed and 5-7m of the topographically highest beds were erosionally truncated to create the famous undulose geometries of the algal facies, commonly known as the “mounds.” Sequence 2 comprises Units 6-10, and shows evidence of an overall relative rise and fall in sea level. Facies of Sequence 2 fill in and drape underlying topography created by Sequence 1. It consists of the following lithofacies: Fusulinid Packstone (FP; 6); Skeletal Wacke-Packstone (SWP, 7); Skeletal Wacke-Packstone-Chaetetes (SWP-C, 7a); Peloidal Mudstone (PM, 8); Quartz Sandstone (QS,9); and Quartz Siltstone (QSt, 10). The build-and-fill model is an enhancement of sequence stratigraphic models. It applies where carbonate strata exhibit subtle paleotopography, were subject to non-optimal carbonate productivity and high-amplitude sea-level changes. The stratigraphic succession in the Lower Ismay algal mounds shows evidence of relative shallowing and deepening during a relief-building phase. A subaerial exposure surface on the top of the algal facies indicates relative deepening was followed by shallowing during a relief-filling phase. The Lower Ismay zone provides an example of build-and-fill geometries that underwent different conditions than typical build-and-fill sequences, ultimately adding to our understanding of the processes that yield build-and-fill geometries.
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Date
2015-12-31
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University of Kansas
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Keywords
Geology, Algal Mounds, Build and Fill, Honaker Trail Formation, Lower Ismay, Paradox Basin, Stratigraphy
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