Structural controls on Quaternary deepwater sedimentation, mud diapirism, and hydrocarbon distribution within the actively evolving Columbus foreland basin, eastern offshore Trinidad

Eastward migration of the Caribbean plate relative to the South American plate has caused lithospheric loading along the northern margin of South America, which is recorded by an 1100-km-long foreland basin which is oldest in the west (Maracaibo basin, 65-55 Ma) and youngest in the east (Columbus basin, eastern offshore Trinidad, 15-0 Ma). The Orinoco River has been the primary source of sediment for the basin since early Miocene. We have integrated approximately 775 km of deep-penetration 2D seismic lines acquired in the area of eastern offshore Trinidad as part of the 2004 “Broadband Ocean-Land Investigations of Venezuela and the Antilles arc Region” (BOLIVAR) project, 8000 km² of shallow industry 3D seismic data, and published industry well data from offshore eastern Trinidad. Active mud diapirism in the Columbus basin is widespread and is related to overthrusting and tectono-sedimentary loading of upper Miocene-lower Pliocene age mud. Analysis of the shallow 3D seismic data reveals the presence of extensive gravity-flow depositional elements on the Columbus basin slope and the deepwater area. These stacked gravity-flow deposits are characterized by mass-transport deposits at the base, turbidite frontal-splay deposits, leveed-channel deposits, and capped by fine-grained condensed-section deposits. Exploration targets in the deepwater area are located towards the center of the Columbus basin, where northeast-trending fault-propagation folds are important Plio-Pleistocene trap-forming elements. Deep basin wells drilled in recent years have proven that turbidites were transported into the deepwater Columbus basin during the Plio-Pleistocene. Analysis of these well results suggests that a deeper oil charge is present within the deepwater Columbus basin area. The primary uncertainty for this variable hydrocarbon system is whether fault or diapiric pathways connect or divert the petroleum charge at depth with shallower reservoir rocks.     

An alternative interpretation of the Cayman trough evolution from a reidentification of magnetic anomalies

Magnetic data were collected during the Wilkes (1973) and Seacarib II (1987) cruises to the Cayman trough. A new interpretation of magnetic data is carried out. An isochron pattern is drawn up from our anomaly identifications. An early Eocene age (49 Ma, Ypresian) for Cayman trough opening is proposed instead of the late Oligocene or middle Eocene ages suggested by previous studies. Our plate tectonic reconstruction is simpler and fits the on-land geology (Jamaica and Cuba) and the tectonics. Our reconstruction shows a southward propagation of the spreading centre between magnetic anomalies 8 and 6 (26 and 20 Ma). The trough width increases by 30 km in this period. The southward propagation of the Cayman spreading centre from the Middle Oligocene to the Early Miocene induced the development of the restraining bend of the Swan Islands, the formation of a 1 km high scarp on the eastern trace of the Cayman trough transform fault (Walton fault) and the formation of a pull-apart basin (Hendrix pull-apart). Magnetic anomalies and magnetization maps give information about the deformation and the rocks. The proposed evolutionary model of the Cayman trough from the inception of seafloor spreading to the present configuration is presented in relation to the tectonic escape of the northern boundary of the Caribbean plate from the Maastrichtian to the Present.     

Differences in vegetation characteristics with shoreline position on a tropical pocket beach

ABSTRACT

Composition and richness of coastal vegetation differs with distance from the water and sheltering by topography. Transition from pioneer beach plants to mature forests is expected to be narrow on low wave energy coasts with a tropical climate favorable to vegetation growth. The goal of this paper is to determine whether vegetation on the beach and foredune will be denser and have a greater number of species and more woody shrubs near the water as wave and wind stresses diminish because of favorable shoreline orientation within a pocket beach. Field data on beach width, beach mobility, dune height, vegetation species, vegetation height and percent vegetation cover were collected in Puerto Rico along six cross-shore transects. Beaches are more stable at transects in the lee of an eolianite barrier and a tombolo. The vegetation gradient there is compressed, with denser, taller, more diverse vegetation and more tree species close to the waterline than at more exposed sites. The lack of mobility of the beaches and dunes and lack of geomorphically significant disturbance events is in contrast to the conspicuous feedbacks between overwash, topography, and vegetation on exposed mid-latitude coasts, revealing the need for more study of low-energy beach environments.     

Bimodal volcanism in northeast Puerto Rico and the Virgin Islands (Greater Antilles Island Arc): Genetic links with Cretaceous subduction of the mid-Atlantic ridge Caribbean spur

Bimodal extrusive volcanic rocks in the northeast Greater Antilles Arc consist of two interlayered suites, including (1) a predominantly basaltic suite, dominated by island arc basalts with small proportions of andesite, and (2) a silicic suite, similar in composition to small volume intrusive veins of oceanic plagiogranite commonly recognized in oceanic crustal sequences. The basaltic suite is geochemically characterized by variable enrichment in the more incompatible elements and negative chondrite-normalized HFSE anomalies. Trace element melting and mixing models indicate the magnitude of the subducted sediment component in Antilles arc basalts is highly variable and decreases dramatically from east to west along the arc. In the Virgin Islands, the sediment component ranges between< 0.5 to  1% in Albian rocks, and between  1 and 2% in succeeding Cenomanian to Campanian strata. In comparison, sediment proportions in central Puerto Rico range between 0.5 to 1.5% in the Albian to 2 to > 4% during the Cenomanian-Campanian interval. The silicic suite, consisting predominantly of rhyolites, is characterized by depleted Al₂O₃ (average < 16%), low Mg-number (molar Mg/Mg + Fe < 0.5), TiO₂ (< 1.0%), and Sr/Y (< 10), oceanic or arc-like Sr, Nd, and Pb isotope signatures, and by the presence of plagioclase. All of these features are consistent with an anatexic origin in gabbroic sources, of both oceanic and arc-related origin, within the sub-arc basement. The abundance of silicic lavas varies widely along the length of the arc platform. In the Virgin Islands on the east, rhyolites comprise up to 80% of Lower Albian strata (112 to 105 Ma), and about 20% in post-Albian strata (105 to 100 Ma). Farther west, in Puerto Rico, more limited proportions (< 20%) of silicic lavas were erupted. The systematic variation of both sediment flux and abundance of crustally derived silicic lavas are consistent with current tectonic models of Caribbean evolution involving approximately perpendicular subduction of the Caribbean spur of the mid-Atlantic Ridge, which was located approximately midway between North and South America until Campanian times. Within this hypothetical setting the centrally positioned Virgin Islands terrain remained approximately fixed above the subducting ridge as the Antilles arc platform swept northeastward into the slot between the Americas. Accordingly, heat flow in the Virgin Islands was elevated throughout the Cretaceous, giving rise to widespread crustal melting, whereas the subducted sediment flux was limited. Conversely, toward the west in central Puerto Rico, which was consistently more remote from the subducting ridge, heat flow was relatively low and produced limited crustal melting, while the sediment flux was comparatively elevated.     

SOIL CONSERVATION,POLITICAL ECOLOGY,AND TECHNOLOGICAL CHANGE ON SAINT VINCENT*

ABSTRACT. A political-ecological perspective is used to analyze soil erosion, conservation, and the peasantry on Saint Vincent in the Eastern Caribbean. Peasants farm areas most susceptible to erosion because of the historical development of property relationships. A soil-conservation effort begun in the late 1930s was part of a broader, British Empire-wide program. Local political-economic conditions and the environmental and technical characteristics of the cropping systems influenced the nature of soil conservation on Saint Vincent. Official colonial discourse about erosion reflected a complex mixture of blaming peasants and recognizing their political-economic constraints.     

Construction of synthetic ocean wave series along the Colombian Caribbean Coast: A wave climate analysis

In this paper a methodology is applied to generate synthetic wave series during mean and extreme conditions. An analysis is carried out that describes mean and extreme wave behavior for several climatic conditions along the Colombian Caribbean Coast. During mean conditions, the most energetic ocean waves are observed during the DJF season for both ENSO phases (El Niño and La Niña) for most of the Caribbean Sea. During the Niño years, there is a reduction in the speed of the north-east trade winds and their associated waves, but only in the DJF and MAM seasons. However, during the JJA season, this situation is reversed with the highest values occurring during El Niño and low values appearing during La Niña. Toward the east around the Guajira region, this general pattern is shown to change significantly. For extreme conditions, the results show a significant influence of extreme events toward the northwest, around La Guajira and the insular zones of San Andres and Providence when compared with other regions along the coast. All of these results (including the synthetic wave series) provide a design and management tool for the successful implementation of any coastal project (scientific or consulting) in Colombia.     

Strike-slip tectonic processes in the northern Caribbean between Cuba and Hispaniola (Windward Passage)

Marine geophysical data including Seabeam, seismic reflection, magnetics, gravimetry and side-scan sonar have been recently collected along the northern Caribbean strike-slip plate boundary between Cuba and Hispaniola, in the Windward Passage area. The analysis of this comprehensive data set allows us to illustrate active strike-slip tectonic processes in relation to the kinematics of the Caribbean Plate. We show that the transcurrent plate boundary trace runs straight across the Windward Passage, from the southern Cuban Margin in the west (Oriente Fault) to the Tortue Channel in the east. The Windward Passage Deep is thus not an active pull-apart basin, as previously suggested. The plate boundary geometry implies that the motion of the Caribbean Plate relative to the North American Plate is partitioned between a strike-slip component, accommodated by the Windward Passage active fault zone, and a convergence component, accommodated by compression at the bottom of the Northern Hispaniola Margin. On the basis of a correlation with onland geological data, an age is given to the stratigraphic sequences identified on seismic profiles. A kinematic reconstruction is proposed that follows the tectonic unconformities recognized at sea and on land (Late Eocene, Early Miocene, Middle Miocene and Late Pliocene). Each one of these tectonic events corresponds to a drastic reorganization of the plate boundary geometry. We propose to correlate these events with successive collisions of the northern Caribbean mobile terranes against the Bahamas Bank. During each event, the plate boundary trace is shifted to the south and a part of the Caribbean Plate is accreted to North America.     

Abundance and Photosynthetic Characteristics of Trichodesmium spp. Along the Atlantic Barrier Reef at Carrie Bow Cay, Belize

Abstract. Trichodesmium abundance and photosynthetic rates were determined across a transect of the Atlantic Barrier Reef at Carrie Bow Cay, Belize in May 1993 and May-June 1994. A fore reef station receiving oceanic water had a Trichodesmium abundance characteristic of the open Caribbean Sea (≤ 10² colonies m^-3 and 10⁴-^-10⁵ trichomes m^-3). Several species were present; however, in all cases a sharp decrease in abundance occurred across the reef to the coastal lagoon. At this latter site, virtually no Trichodesmium was present, although a substantial bloom of the diatom Proboscia alata occurred in 1994.
Maximum photosynthetic rates and the initial light-limited slope a were significantly lower in offshore stations than in reef top populations. The increased photosynthetic rates and light-harvesting ability in reef-top populations, as well as the lack of significant photoinhibition, indicated that the Trichodesmium abundance decrease was probably not due to a decline in photosynthetic capacity. Grazing and physical removal rather than a physiological decline were likely mechanisms operating to reduce Trichodesmium abundance in these regions. These data suggest that the Belizean barrier reef is a sink for Trichodesmium transported in from offshore regions.     

Geological interpretation of combined Seabeam,Gloria and seismic data from Anegada Passage (Virgin Islands,north Caribbean)

The Anegada Passage (sensu lato) includes several basins and ridges from Southeast of Puerto Rico to the corner of the Virgin Islands Platform. Seabeam (Seacarib I) and Gloria long-range sidescan sonar surveys were carried out in this area. These new data allow us to propose an interpretation of the Anegada Passage.Most of the features described are related to wrench faulting: