Late Mississippian Lime Mud Mounds, Pitkin Formation, Northern Arkansas: ABSTRACT

Abstract

Carbonates deposited under shallow, open shelf conditions during the Late Mississippian in northern Arkansas exhibit numerous discrete to coalescing lime mud mounds up to 20 m (65 ft) high and tens of meters in diameter. The mounds are composed of a carbonate mud core, typically with fenestrate texture, entrapped by a loosely organized framework dominated by cystoporate bryozoans and rugose corals in the lower part, and by blue-green algae and cryptostomous bryozoans in the upper part. Disarticulated crinozoan detritus is common throughout the core, suggesting that these organisms also contributed to entrapment of lime mud. During deposition, the mud core was indurated enough to support and preserve vertical burrows. Also, rubble of core mudstone is found on the flanks of some mounds, suggesting some erosion. Intermound lithology is a shoaling-upward sequence dominated by oolitic and bioclastic grainstones and packstones. Shale is also present in minor amounts. The Pitkin mounds, interbedded with these intermound sequences, developed contemporaneously with them. Depositional relief was probably less than 3 m (10 ft). The mounds expanded laterally during periods of quieter water; their growth was impeded during times of higher energy. Contacts of the mound and intermound lithologic characteristics are sharp, truncating surfaces. Mound deposition ended with the onset of high energy conditions throughout the region. End_of_Article - Last_Page 503------------