Sedimentology and Ichnology of a Pleistocene Unconformity-Bounded, Shallowing-Upward Carbonate Sequence: The Ironshore Formation, Salt Creek, Grand Cayman

Abstract

The Pleistocene Ironshore Formation around Salt Creek, Grand Cayman, is an unconformity-bounded stratigraphic sequence formed of oolitic limestones that accumulated in a lagoonal setting subjected to progressively increasing energy conditions. The limestones of the Ironshore Formation at Salt Creek, which are up to 7 m thick, accumulated in a lagoon immediately behind the main reef crest. In this area, the upper part of the formation contains channel diastems that can be used to divide it into lithosomes I, II, and III. The lower part of lithosome I comprises scattered patch reefs surrounded by oolitic limestones. The upper part of lithosome I, formed after the demise of the patch reefs, comprises oolitic limestone that accumulated in relatively quiet water conditions. A period of intense erosion then produced two channels that were separated by a bank with a relief of at least 1.5 m, and subsequently initiated deposition of large lithoclasts on the channelfloors. Lithosome II is formed of oolitic limestones that were deposited under moderately high energy conditions in the confines of the channels. This phase of deposition was terminated by a second period of erosion that cut into the channel deposits of lithosome II in the northern part of the area. As with the lower channels, large lithoclasts were also deposited on the floor of this channel. High-energy conditions then resulted in the deposition of cross-bedded oolitic limestones in the confines of the channel. Ichnological evidence shows that the channel diastems, which formed initially as a result of erosion, evolved into firmgrounds prior to the deposition of the sediments over them. Analysis of the shallowing-upward sequence in the Ironshore Formation exposed near Salt Creek on Grand Cayman demonstrates the internal depositional complexity of unconformitybounded sequences, and shows how integrated lithological and ichnological analyses can provide valuable information about discontinuities in a sequence.