Holocene Overprints of Pleistocene Paleokarst: Bight of Abaco, Bahamas

Abstract

Unconformable facies relationships at the Holocene/ Pleistocene interface in the Bight of Abaco lagoon, northern Bahamas, have been investigated by high- resolution seismic profiling, rock coring, and excavation of buried bedrock surfaces. Lateral variation in the expression of a single lagoonal unconformity can be explained in terms of different genetic and preservational processes affecting the Pleistocene subaerial exposure surface during Holocene sealevel rise.

The dished bedrock surface beneath the Bight of Abaco has been divided geomorphically into a central depression, a transitional slope, and a marginal terrace. Upon Holocene sealevel rise, irregular karstic central depressions (dolines) are first wet by freshwater, preserving them beneath peats or paleosols. Along the transitional bedrock upslope, pedogenic caliche crusts are buried and preserved intact during schizohaline (mixed salinity) conditions. Continued sealevel rise ultimately results in the stable marine environment present within the lagoon today. These conditions initiate marine deposition and bioerosion at the widening lagoon margins, both of which drastically change the burial environment and preservation of the subaerial exposure surface there. A seismically observed break in bedrock slope characterizes the marginal terrace. Bedrock excavation suggests that it is a result of the initiation of coastal marine bioerosion and consequent truncation of the original subaerial exposure surface. Holocene flooding of a silled interior bedrock depression has created three distinct preservational environments, resulting in the burial of three demonstrably different, concentrically zoned Holocene/Pleistocene unconformities at the contact.

Differences in the final expression of paleokarstic unconformities (i.e., karstic, caliche, and bioeroded) are the combined product of the exposure environments which produced them and the burial environments which altered or preserved them. Temporal and spatial differences in the genetic and taphonomic environments of paleokarstic unconformities must surely complicate attempts at strati- graphic correlation, especially when glimpsed in intermittent outcrop. An actualistic model is proposed for central to marginal changes in substrate preservation beneath “dished” carbonate lagoons as a function of sealevel change. Predictable changes in the preservation of ancient subaerial unconformities can yield useful information on antecedent platform topography, sealevel history, basin geometry, and the paleoceanography of the evolving lagoon.