Depositional Environments of Pennsylvanian Upper Strawn Group in McCulloch and San Saba Counties, Texas: ABSTRACT

Abstract

Upper Strawn Group (Desmoinesean) represents a transition to fluvial facies from progradational deltaic facies. The lower part of the upper Strawn is composed mostly of horizontally bedded, fine-grained sandstones and shales of a distal delta-front origin. These sandstones and shales exhibit foreset bed dips of up to 15°. In addition to the dipping foreset beds, the delta-front facies on occasion contain small listric normal faults, resulting from periodic higher rates of sedimentation. The middle parts of the upper Strawn consist predominantly of massive, fine to medium-grained, mature sandstones which represent distributary-mouth-bar deposits, as well as other proximal delta-front deposits such as distributary channels. The upper part of the upper Strawn consists o fluvial trough cross-bedded sandstones and chert-pebble conglomerates. These overlie the deltaic facies and indicate the final stages of upper Strawn deposition. The upper Strawn is overlain by the Adams Branch limestone and shales which represent marine transgression and subsequent shallow-marine deposition. The upper Strawn Group in McCulloch and San Saba Counties, Texas, represents continued filling of the Fort Worth basin during Desmoinesean time. The upper Strawn overlies the lower Strawn, an older, deeper water facies, in most parts of the study area. The upper Strawn overlies the Atokan age Marble Falls Limestone in an isolated section of the study area due to its position there on the Concho arch. End_of_Article - Last_Page 489------------