ABSTRACT

The Marble Falls Group, a Lower Pennsylvanian limestone exposed at the margins of the Llano uplift in central Texas, was deposited on a carbonate shelf bordered by the Fort Worth basin. Outcrops near the basinward margin of the shelf reveal a depositional cycle containing a regressive sequence, a low-relief unconformity, and a transgressive sequence.

Minor cycles within the transgressive and regressive sequences include inner shelf successions of facies capped by thin intervals of the outer shelf facies. Vertical progressions of inner shelf facies, algal bank to calcarenite bank to tidal bar-tidal flat, were clearly the product of regression. Thin capping intervals of the outer shelf facies contain open marine clay and carbonaceous matter mixed with siliceous sponge spicules, micrite and pellets. Incursions of the outer shelf facies onto inner shelf facies are judged to be a product of transgression.

The regressive sequence exhibits a striking down-to-basin shift of progressively younger minor cycles, interpreted to indicate net regression interrupted by minor transgressions. The transgressive sequence exhibits similar cyclic tendencies and is interpreted to represent net transgression interrupted by minor regressive episodes.

There is striking similarity between the interrupted patterns of transgression and regression in the Marble Falls cycle, and the discontinuous nature of ice sheet advances and retreats in Pleistocene climatic cycles. Estimates of relative sea level changes necessary to account for Lower Pennsylvanian facies patterns are in the same range as those resulting from Pleistocene climatic cycles. After carefully considering all cycle-generating mechanisms it is concluded that the Marble Falls cycle was most likely produced by glacio-eustatic changes.