- Copyright © 2017, SEPM (Society for Sedimentary Geology)
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 ∼ 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.
Based on their relatively uniform size, variation in iron-oxide content, and association with small ferruginous concretions, the aggregates formed by reworking of associated vertic paleosols. In the absence of discrete smectite, their formation probably reflects a minor swelling-clay component in illite, Fe-rich chlorite, and mixed-layer clays, as indicated by glycolation results. Deficiency in smectite is probably a result of parent-rock composition, rather than burial illitization in this shallowly buried formation.
In modern floodplains, aggregates typically lose their pelleted texture during shallow burial. Their excellent preservation at the study site is attributed mainly to the lack of swelling clays in the mud aggregates and their rapid incorporation into a migrating point bar, set within a “compartment” of indurated paleosols that limited compactional effects. Some early cementation by dolomite, gypsum, and barite of the aggregate-bearing and associated beds would have enhanced rigidity.