SEDIMENTOLOGY OF THE CHANGUINOLA PEAT DEPOSIT - ORGANIC AND CLASTIC SEDIMENTARY RESPONSE TO PUNCTUATED COASTAL SUBSIDENCE

An extensive peat deposit on the Caribbean coast near Changuinola, Pan ama, has developed in an area subject to periodic earthquake-driven co seismic subsidence, Thick, low-ash, low-sulfur peat is accumulating im mediately behind an aggrading and prograding barrier system and adjace nt to a flood-prone, sediment-laden river, Measurements of changes in local sea level as a result of a recent (April 1991) earthquake reveal 30-50 cm of subsidence, greatest at the southeastern extent of the st udy area, where the peat surface is submerged to a depth of 3 m beneat h the shallow waters of Almirante Bay, Transgression is proceeding fro m southeast to northwest, parallel to the trend of the coast and the l ong axes of both the peat deposit and the major sand bodies, In the ea stern part of the deposit, the effects of sea-level rise are evident i n the degree of humification, mineral matter, and sulfur content of ma ngrove and back-mangove peats offshore and immediately adjacent to the marine margin, and in peats associated with brackish tidal channels t hat drain the deposit, However, most of the deposit shows no indicatio ns of marine influence, even though approximate to 40 % of the peat is below present sea level, The western section of the deposit has evolv ed from low-lying palm swamps, which originated in swales on the barri er bar, into an oligotrophic bog plain with a water table elevated 6.7 5 m above sea level, As the mire evolved, transitions in vegetation re sulted in transitions in peat types, Highly humified forest-swamp and palm-swamp peats underlie and surround well-preserved, fibric sedge pe ats, and create a partial hydrological bounding surface that restricts subsurface drainage from the central bog, The high water table and el evated topography of the mire and the low permeability and erosion res istance of the dense, moody peat effectively insulate most of the depo sit from both elastic influx and the extensive intrusion of rising mar ine waters, It is evident that thick peat, and hence coal, deposits ca n accumulate due to tectonically driven, punctuated subsidence without leaving a record of high elastic input within the peat, even immediat ely adjacent to environments of active clastic deposition.