Beata Ridge Research Articles

Deep-Sea Pleistocene Biostratigraphy

The first detailed paleontological analysis of a deep-sea pistoncore from the Caribbean Sea has been completed. The core, P6304-8, was raised from 3927 meters, east of Beata Ridge at 14 degrees 59'N, 69 degrees 20'W. Formerly, stratigraphic works in this area were based on studies of paleotemperature, measured by the oxygen isotope mass spectrometry method, or on micropaleontological analysis by means of rapid or cursory examinations. For core P6304-8, samples for foraminiferal analysis were taken at 10-centimeter intervals and split into smaller samples containing an average of 710 individuals (smallest sample, 517 individuals); all individuals were then identified and counted. By use of data derived from populations of this size, a statistical reliability was insured within a 5 percent limnit. Temperature oscillations, the best method of portraying Pleistocene stratigraphy, were shown by using ratios of the relative abundances of tropical and subtropical planktonic foraminifera to those found in temperate and cooler waters. These ratios correlate well with existing paleotemperature measurements for the same core, obtained by the oxygen isotope mass spectrometry method.

Geophysical measurements in the western Caribbean Sea and in the Gulf of Mexico

The data from 48 seismic refraction profiles in the western Caribbean Sea and in the Gulf of Mexico are presented in the form of structure sections crossing the Colombian basin, Nicaraguan rise, Cayman trough, Cayman ridge, Beata ridge, Yucatan basin, Campeche bank, and Sigsbee deep. The Cayman trough has a remarkably thin crust, which suggests that it is a tensional feature. Although parts of the basins have a relatively thin crust, similar to the oceanic type, the shallower areas are intermediate or almost continental in structure. In the Gulf of Mexico the main basin is similar to typical ocean basins in structure except that the high-velocity crust is overlain by very thick sediments. The depth to the mantle is appreciably greater in the Gulf than in an ocean basin. This may be partly the result of loading by the sediments, but large scale tectonic activity is a more likely cause. The Sigsbee escarpment, the northern boundary of the main basin, appears to be the surface expression of a fault or sharp flexure in the layers beneath the unconsolidated sediments.

The Relationship between Paleotemperature and Carbonate Content in a Deep-Sea Core

Core A172-6 taken at a depth of 4,160 meters from the eastern extension of the Beata Ridge by the Lamont Geological Observatory has been spectrochemically analyzed for selected major and minor elements. A detailed point-by-point comparison of the data thus obtained has been undertaken with reference to published paleotemperature data for the same core at correlative interval of 10 cm. spacing throughout a core length of 9.35 meters. Since the carbonate content of an ancient sediment is often cited as a clue to the temperature of the sea water at the time of the sediment's formation, a study of the paleotemperature and carbonate content relationships in this core has been undertaken for the purpose of examining the validity of this concept for a sea which, during the time of most extreme glaciation, exhibited a paleotemperature range analogous to those which exist in modern tropical seas. Data are presented which tend to indicate that no correlation exists between carbonate content and paleotemperature in this core. Spectrochemical analyses have been performed for eleven elements, and these data are presented in tabular form.

Tectonic Relations of North and South America

The Pacific and Atlantic marginal belts of orogeny in Paleozoic time are believed to merge in central and southern Mexico and then as a single belt to swing eastward through Central America approximately to Jamaica and Hispaniola. Veering sharply southward from Hispaniola the belt includes the present submerged Beata Ridge and connects with the metamorphic core of the Venezuelan and Colombian Andes. The major Mesozoic belt of orogeny is believed to extend down the peninsula of Baja California, through the Sierra del Sur of Mexico to the Costa Rica-Panama Isthmus and the Colombian and Ecuadorian Andes. It is marked throughout by great granitoid batholiths. Another belt of late Jurassic and Cretaceous orogeny branches eastward in Central America from the main belt and follows through the Greater and Lesser Antilles to Trinidad and Venezuela to join the main belt in Colombia. This belt makes a tight U-shaped pattern and is believed to mark two tectogenes for the most part, both of which formed in the late Mesozoic time and probably in succession one after the other. A third tectogene from Puerto Rico around the Lesser Antilles to the Leeward Islands is active to-day and probably had a beginning in Eocene time. It lies outward from the late Mesozoic tectogenes. The available geophysical data in the form of gravity anomalies and seismic observations are related to the geological information in making the analysis of the tectogenes. The eastern Caribbean Basin is believed to be due to middle and late Tertiary subsidence, and the western Caribbean Basin is believed to be due to Cretaceous and early Tertiary subsidence. The Gulf of Mexico is postulated to have begun to subside in Permian time. From the sequence of events described for each of the basins in the Mexican-Caribbean region, as well as for the Banda Sea of the Dutch East Indies, an orogenic cycle is postulated as follows: (1) general epeirogenic uplift of a central region for a long time, and the shedding from it of large amounts of debris to partly surrounding basins; (2) compressional deformation in the partly surrounding basins to form a belt of major orogeny; and (3) subsidence of the former region of uplift, with the creation of a basin having deep water in places. The marginal areas of the new basin soon become sites of heavy sedimentation. From this tectonic setting a theory of orogenesis is devised which proposes asymmetrical convective flow in the mantle as the causative mechanism. The theory is applied to the Dutch East Indies and the Peruvian Andes as well as the mediterranean region between North America and South America.