DISTRIBUTION AND DEPOSITIONAL HISTORY OF SOME PRE-LAGOONAL HOLOCENE SEDIMENTS IN THE CIÉNAGA GRANDE DE SANTA MARTA, COLOMBIA

Abstract

An investigation of the pre-lagoonal Holocene sediments from beneath the Ciénaga Grande de Santa Marta (Colombia) was undertaken for the purpose of better understanding the paleogeographical and paleoecological conditions of this region before the formation of the lagoon proper. Core and grab samples were collected from the lagoon for stratigraphic observation and analysis in the laboratory. Beneath the recent lagoonal sediments a deposit of peat with intercalated sand and clay was discovered. Its surface lies at a depth of about 2 m below the hydrographic zero level of the lagoon. Three peat samples, obtained from the upper portion of this deposit, were subjected to micropetrographic and pollen analyses. These analyses revealed that, prior to marine inundation, this region was a coastal marsh-swamp complex similar to the Everglades-mangrove region in southern Florida (U. S. A.). Sample "f" , taken closest to the present sea, represents a red mangrove (Rhizophora mangle) dominated zone, which was relatively exposed to tides and waves, so that the energy was great enough to sweep the sediment surface clean of litter. This implies that the Isla de Salamanca, an almost continuous beach barrier between the present-day lagoon and the Caribbean Sea, must not have been as prominent a barrier as it is today, but that a larger inlet existed near the site of formation of this particular peat sample. The next most inland sample " g" represents a mixed mangrove zone which was somewhat less exposed to current scour and which was presumably situated behind and protected by the former zone. The third sample "h" , taken the furthest inland, represents ponded conditions within a slightly brackish to fresh water marsh dominated by leatherleaf fern (Acrostichum aureum) and sedges. This relatively stable zonation of vegetational environments from marine mangrove in the north to fresh water marshes in the south briefly before the termination of peat formation suggests that the transgression which formed the lagoon was so rapid that marine floral communities had no time to migrate inland and cover more of the submerging swamp surface. This assumption is supported by C14 dates which show that the uppermost peat layers were formed approximately 2 400 years ago in the north and 1 900 years ago in the south, so that the transgression over the swamp should not have taken longer than 500 years. It is recognized, however, that the small number of analyzed samples does not permit far-reaching conclusions. Four different species of molluscs encountered in peat and clay samples from a few locations in the northern and southern Ciénaga are typical brackish water species. This is evidence for the early existence of small low-salinity lagoons and creeks within the marsh-swamp setting. Some additional stratigraphic information was obtained by probing with a steel rod capable of penetrating the recent lagoonal sediments as well as the peat. With reference to the water level a shorter depth down to a "firm substrate" (dense sand or indurated clay) was encountered in the southern part of the lagoon than in the rest of the lagoon. This "substrate high" probably represents a submerged lobe of a former subdelta of the Magdalena River. The common association of peat with layers of sand and clay in this region also points to a nearby source for clastics. In general the discharge of sediments into the swamp was sporadic, with long periods during which the rate of clastic sedimentation was low enough to allow the accumulation of relatively pure peats.