PEDOGENIC MUD AGGREGATES PRESERVED IN A FINE-GRAINED MEANDERING CHANNEL IN THE LOWER PERMIAN CLEAR FORK FORMATION, NORTH-CENTRAL TEXAS, USA

摘要

The Lower Permian Clear Fork Formation of north-central Texas is among few examples from the rock record where mud aggregates are well-preserved as the main sediment component of a meandering channel. This rare occurrence provides an excellent opportunity to evaluate the origin and depositional setting of aggregates, as well as the conditions responsible for their preservation. At the study site, lateral-accretion deposits define an exhumed point bar composed of thick mudstone layers rich in aggregates, interbedded with thin layers of ripple cross-laminated sandstone. Petrographic analysis indicates that red-brown and gray aggregates constitute > 68 of the mudstones, with minor quartz, feldspar, ferruginous grains, and dolomite rhombs. When size estimates are corrected for 2D sections and for similar to 17 flattening during compaction, the aggregates are of fine-sand size and more than twice the size of associated quartz grains, which were probably size-limited on the distal plain. Despite the difference in grain size and density, the aggregates and quartz populations were both entrained at very low velocities ranging from 1.2 to 1.5 cms(-1).