Morphological Evolution of Cambrian Algal Mounds in Response to a Change in Depositional Environment

Abstract

An evolutionary trend in algal mound morphology within the Morgan Creek Limestone, upper Cambrian of central Texas, is associated with a change in depositional environment. Morphological modification of the mounds is a response of the algal communities to changes in water depth and turbulence. This change occurs through a stratigraphic interval approximately 20 m thick. The lower part of the Morgan Creek Limestone exhibits substantial evidence of having been deposited in a more turbulent environment than the upper part. This is indicated by an upward decrease in abundance of oolite bodies; an increase in micrite and corresponding decrease in spar; a decrease in cross-stratified deposits upward; and, in the middle of the section, the first appearance of a green alga. The lowest mounds, stratigraphically, are discrete club-shaped, simple, highly arched, nonbranching, concentric structures. These mounds, which grew in a high energy environment during deposition of the lower part of the Morgan Creek, have maximum dimensions of 0.5 m in thickness and 0.75 m in diameter. In the middle Morgan Creek mounds are larger, 0.25 to 1 m thick and 0.3 to 1.7 m in diameter, and have a complex digitate internal structure. Near the top of the Morgan Creek Limestone is a horizon of the largest-sized algal mounds, biconvex lenses up to 1.7 m thick and 8 m in diameter. The over-lying strata contain some flat, algal-laminated structures. This evolution in mound form, a decrease in height to width ratio and from simple to complex internal structure, is associated with a decrease in water turbulence and a shift from shallow marine to intertidal to supratidal site of deposition. This environmental response demonstrates that changes of mound morphology can be useful in interpreting depositional environments.