Detailed field and laboratory observations are the basis for the description of 13 lithofacies in the Middle Ordovician Chickamauga Group in Jefferson County, Alabama. These lithofacies, including stromatolitic, fenestral and bioturbated mudstones, and varieties of skeletal and peloidal wackestones, packstones and grainstones, represent deposition in seven major environments: high intertidal, low intertidal, tidal channel, subtidal wave baffle, subtidal bar, subtidal bank, and subtidal level bottom. Middle Ordovician deposition began on the extremely irregular karstic surface of the Cambro-Ordovician Knox Dolomite. A lag deposit of reworked dolomite and chert clasts (Attalla Member) is overlain by intertidal deposits. High and low intertidal lithologies vary laterally in hickness in response to deposition over highs present on the irregular Knox surface. With continued transgression, shallow-water facies were succeeded by a suite of subtidal deposits composed dominantly of level-bottom lithologies deposited below wave base. Changes in sedimentation and/or subsidence led to a gradual shallowing and deposition above wave base. Coarse-grained, high-energy tidal bars and skeletal bank deposits dominate this part of the sequence. In the southern part of the study area, subtidal level-bottom lithologies dominate the upper part of the Middle Ordovician sequence. To the north, these lithologies grade laterally into shallower subtidal and intertidal lithologies. The sequence is unconformably overlain by the Upper Ordovician Sequatchie Formation.

End_of_Article - Last_Page 1433------------