Distribution of Facies, Depositional, and Diagenetic History of Knowles Limestone (Cotton Valley Group-Late Jurassic), De Soto Parish, Louisiana

A. K. M. Mahbubullah

The Knowles Limestone in northwestern Louisiana was deposited in a warm, very shallow sea on a carbonate ramp. Depositional and diagenetic environments suggest a tropical to subtropical humid climate. Four major depositional settings were present: a restricted tidal flat, a restricted to open lagoon, a reef flat, and a reef core. Restricted tidal-flat deposits include algal-laminated mudstones, wackestones, and bindstones; dolostones; and mudstones where algae and mollusks were abundant with minor echinoderms and bryozoans. Mostly wackestones and rarely packestones were deposited in the lagoon. Oysterlike bivalves and calcareous algae were very common in the lagoon, with coral, hydrozoans, echinoderms, and bryozoans occasionally abundant. Bioturbation and dolomitization a e very common in the tidal flat and lagoonal deposits.

Principal contributors to the reef framework were corals, stromato-poroids, and encrusting algae which reflect ecological succession. In the reef-flat environment, oncoidal-skeletal wackestones to packstones were deposited. Rarely, they attained bindstone fabric encrusted by codiacean algae. Reef development was ended by prograding tidal-flat deposits.

The Knowles sediments display diagenetic signatures from marine phreatic, meteoric phreatic, fluid mixing, subaerial, basinal fluid expulsion, and tectonic stages of diagenetic alteration. Among the Knowles sediments, supratidal dolostones possess best porosity and permeability, but residual hydrocarbon saturation is nil.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.