Stratigraphic architecture of climate influenced hyperpycnal mouth bars

Abstract

AbstractInvestigation of the process sedimentology of mouth bar deposition demonstrates a link to the seasonality associated with abrupt climate warming events during the early Eocene. While a relationship between fluvial sedimentology and climate events has been established previously, such documentation within mouth bar deposits is missing. Deltaic mouth bars of the Douglas Creek Member of the Green River Formation in the Uinta Basin record a fluvial–lacustrine response to abrupt warming events associated with the Early Eocene Climatic Optimum. This study ties the observed sedimentology of these mouth bars and their deposition via hyperpycnal currents to these climatic events. Detailed measured sections and analysis of stratigraphic architecture of outcrops in the eastern Uinta Basin document the sedimentology and stratigraphy of delta mouth bar complexes. Internal structures include climbing ripple cross‐lamination, convolute bedding, and those associated with transitional to upper‐flow regime bedforms such as planar laminations with normal grading and convex‐upward stratifications. The dominance of upper‐flow regime bedforms as well as interpreted long run out distances suggests that density currents deposited mouth bars tens of kilometres from the shoreline under Froude supercritical flow conditions. The highly seasonal climate regime of the early Eocene, capable of producing high‐sediment yield flooding events, likely provided the mechanism for hyperpycnal current formation within a relatively freshwater lake and controlled the timing of mouth bar deposition. Discrete lacustrine carbonate‐dominated packages separate periods of episodic fluvial discharge, recording reduced fluvial deposition between warming events. The established link between the occurrence of upper‐flow regime bedforms and the timing of climate warming events suggests that these climate conditions may be important for their formation and preservation. The study has implications for expanding existing mouth bar depositional models to include climatically controlled upper‐flow regime bedforms.