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摘要
The stratigraphic architecture of carbonate successions that develop on geographically isolated islands reflects the balance between sea-level highstands, lowstands, and tectonic activity. This is readily apparent in the Tertiary carbonate sequences on the Cayman Islands that are formed of the Bluff Group, which includes the Brac Formation (Lower Oligocene), Cayman Formation (Middle Miocene), and Pedro Castle Formation (Middle Pliocene). These strata are overlain by the Pleistocene Ironshore Formation. The unconformities that define the boundaries between these formations are variable because some developed during one cycle of past erosion, others developed through two or more past erosional cycles, and some are still developing today. Some unconformities, like the one at the top of the Cayman Formation, are geographically variable because they underwent different developmental histories in different areas. The challenge with architectural complex successions, like those on the Cayman Islands, is that of deciphering the impact of sea-level changes as opposed to tectonic influences. During sea-level lowstands, the older carbonate successions were exposed on land and modified by surface and subsurface karst development. At the same time, marine erosional processes affected the coastal areas. Surface karst modification, which commonly produced rugged topographies with erosional relief at least 62 m, was controlled largely by rainfall, runoff, and stratal dip. Weathering on Grand Cayman at the end of the Miocene, for example, produced a dish-shaped topography with elevated peripheral rims. In contrast, uplift of the east end of Cayman Brac between the Late Pliocene (3.6 Ma) and similar to 400 ka, elevated the basal part of the Cayman Formation 33 m above sea level. Subsequent karst development, which is still ongoing today, removed most of the Cayman Formation on the eastern part of the island and produced peripheral rims that are higher than those on Grand Cayman. During some lowstands, like that between the Late Pliocene and similar to 400 ka, coeval coastal erosion led to the development of coastal benches that cut into the older carbonate strata. The combination of karst development in the islands' interiors and coastal erosion produced complex, rugged topographies that strongly influenced patterns of deposition during the following highstand. The complex stratigraphic architecture of the carbonate successions on the Cayman Islands reflects the variable impact of tectonics, karst development, and coastal erosion that was associated with each lowstand-highstand cycle.
- 出版日期2014-5-15
