Evaporite Depositional Fabrics in the Deeply Buried Jurassic Buckner Formation, Alabama

Abstract

ABSTRACT The Upper Jurassic Buckner Formation contains evaporites (anhydrite) buried to depths of up to nearly 6,000 m in southwestern Alabama. Although these evaporites and the associated sediments have been deformed and mineralogically altered, some primary depositional textures and fabrics remain. This establishes the Buckner Formation as the most deeply buried succession of evaporites from which primary features have been reported. A cored section of the Buckner Formation contains repetitive depositional sequences, from 1.5 m to 5.5 m in thickness, of 1) dolostone facies (shallow, restricted lagoon) overlain by 2) nodular anhydrite facies (sabkha). Within the nodular anhydrite facies, layers containing nodules surrounded by siliciclastic sediment and finely crystalline dolomite are interpreted as having formed by displacive growth of gypsum or anhydrite at or above a sabkha water table. Other intervals within the nodular anhydrite facies consist of vertically oriented and elongated nodules thought to be pseudomorphs after gypsum that originally formed by subaqueous crystal growth. Each of the Buckner Formation sequences represents the outbuilding of an arid shoreline with progradation of sabkha evaporites and siliciclastics over lagoonal carbonates in a manner similar to the Holocene sequences of the Persian Gulf and the northwest Gulf of California. The Buckner sequences that contain the vertically elongated anhydrite nodules differ from the normal Persian Gulf record of supratidal sabkha deposition in that these pseudomorphs are interpreted as having formed as gypsum in temporary ponds of a sabkha proper, a gypsum pan.