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

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.