STRATIGRAPHY, SEDIMENTOLOGY, AND GEOLOGIC EVOLUTION OF EASTERN TERRABA TROUGH, SOUTHWESTERN COSTA RICA (TECTONICS, FOREARC BASIN)

摘要

Sedimentary rocks of eastern Terraba Trough, southwestern Costa Rica, were deposited in a fore-arc basin developed at an ocean-ocean convergent boundary. The basin developed in Middle to Late Eocene when the Farallon Plate began its subduction beneath the Caribbean Plate. Shallow-water carbonates of the Brito Formation were deposited on shoals of basement blocks. These were surrounded by deeper-marine areas in which volcaniclastics and carbonate debris accumulated. The Brito Formation consists of algal-foraminiferal packstone to grainstone, rudstone, and rare wackestone formed in force-slope, carbonate buildup, and open platform environments in a warm, tropical sea.;The environmental conditions remained unchanged during the Oligocene. Within the study area, the Eocene Brito Formation is overlain directly by rocks of the Upper Oligocene Rio Claro Member of the Terraba Formation, a newly defined carbonate unit. It is composed of rhodolite and bioclastic grainstone deposited in a minimum water depth of 6 meters. A combination of little subsidence, mild volcanism to produce only small amount of clastic debris, and possible erosion at about 30 Ma during a global drop of sea level may be responsible for the absence of Lower Oligocene rocks in the study area.;After the deposition of the Rio Claro Member, the study area subsided rapidly to become a trough possibly deeper than 2,000 meters. Trace fossils, the association of sedimentary structures, and Markov chain analysis of clastic lithofacies indicate that sedimentation occurred in deep water from sediment gravity flows.;In Early to early Middle Miocene, coarser sediments and thicker sand units containing coal fragments, Pecten, and Turritella, became more abundant, suggesting that the basin was gradually filled. Microprobe analysis of sediments indicates that the source for the detrital material was a basaltic andesite to basaltic magmatic arc.;The results of this study indicate that the timing and degree of subsidence of the fore-arc basin, the vertical variations in lithology, and the geologic evolution of eastern Terraba Trough are directly related to the Mesozoic and Tertiary tectonic evolution, in particular, the variations in convergence rate between lithospheric plates in this part of Central America and the eastern Pacific Ocean.