Sedimentology of the Fluvial Systems of the Clear Fork Formation in North-Central Texas: Implications for Early Permian Paleoclimate and Plant Fossil Taphonomy

Abstract

The Lower Permian Clear Fork Formation of north-central Texas is a ~350 m thick redbed deposit with extensive paleosols and well-preserved plant and vertebrate fossils. Deposited on the western equatorial margin of Pangea under (semi)arid conditions on the stable Eastern Shelf of the Midland Basin, the formation experienced shallow burial, which minimized diagenetic effects and allowed the preservation of paleoclimatic indicators. This study investigates the spatial and stratigraphic variations of the alluvial systems and provides a parallel assessment of Early Permian paleoclimatic trends established from previous studies. Four fluvial styles were identified, representing fine-grained meandering and coarser ephemeral sandbed channels. The meandering channels exposed in cliff sections and as exhumed point bars represent a continuum between sustained lateral accretion of bedload composed of quartzose sediments and reworked pedogenic mud aggregates, and oblique accretion of suspended sediments. Swept ripples were formed as water levels declined rapidly to near-dryness and as overbank floodwaters re-entered the channels downstream. Mud aggregates are unusually well-preserved in some channel bodies due to rigid surrounding grains and cements, rapid burial within the migrating point bar, and shallow long-term burial. Abandoned channel fills contain variegated laminated mudstone with well-preserved plant leaves derived from the vegetated riparian zone. The ephemeral channels are represented by tabular sandstones that cut across the landscape and show evidence for strong but fluctuating flow. In the formation, cements and nodules change upward from mainly calcite and ankerite in the lower unit to dolomite and gypsum with minor celestine in the middle and upper units, implying increasingly saline groundwater and supporting progressive regional aridification, which is also evident in other Late Paleozoic studies. In the meandering channel bodies, upper flow regime bedforms are confined to the lower unit, and the occurrence of plant and animal fossils decreases upsection. However, tabular sandstones and avulsive meandering bodies in the middle unit imply more humid periods with higher discharge. Many features of the Clear Fork Formation are similar to the Channel Country of central Australia, making it an appropriate modern analogue.