FLORIDA BAY: MODERN ANALOG FOR LOFER CYCLOTHEMS?

Cyclic sedimentation has deep roots in the Midcontinent. It is embedded in the rocks and was extensively studied by R.C. Moore, among others. One offshoot was A.G. Fischer's interpretation of Lofer cycles in the Triassic Dachstein Limestone of the Northern Limestone Alps. His seminal 1964 paper provided sound interpretations of the succession of depositional environments represented in the cycles as well as pioneering analytical approaches. Recent work on the Dachstein has shown considerable complexity, predictably, in cycle development and also striking lateral variability within the type Lofer cycles. Shoaling upward, deepening upward (Fischer's idealized “Lofer cyclothem”), and symmetrical cycles are represented, as well as many truncated cycles and rhythmic alternations. Fully 22% of all depositional units pinch out laterally and another 13% thin significantly.

Both the lateral variability and the basic sequence of depositional environs suggest analogies with the modern sedimentary record of Florida Bay. The substrate of Florida Bay is a microkarst surface of Pleistocene limestone. The bulk of the muddy molluscan carbonate accumulates in subtidal mudbanks, capped locally by small supratidal islands. These relations generate an asymmetric transgressive/regresssive cycle with but very limited lateral extent. This analog may elucidate two salient features of the Lofer cycles: the sequence of deposition and the lateral variability