Abstract Well and seismic data were combined with existing radiometric data and 1328 new laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb zircon ages from igneous, sedimentary and volcaniclastic rocks in three wells to determine the origin and evolution of key crustal terranes, and the timing of initial rifting and synrift deposition in the southeastern Gulf of Mexico and western Bahamas. These data reveal that several southeastern Gulf of Mexico crustal terranes, distinguished by their zircon age distributions and geochemistries, were amalgamated into a single terrane of mixed Pan-African and Amazonian crustal affinity (the newly named West Florida Terrane) during the formation of Pangaea. Sinistral offset along the Florida Transfer Zone (FTZ) during the Late Triassic–Middle Jurassic was likely to have been facilitated by crustal heterogeneity at the boundary between the West Florida and Suwannee terranes, a prime area for reactivation during the break-up of Pangaea. Initial extension, igneous activity and synrift deposition in the region began during the Middle Triassic in the western Bahamas and offshore North Florida, and rapidly expanded into the South Florida Basin by the Early Jurassic. Peak igneous activity and accommodation in the region was diachronous from north to south, beginning in the north during the Early Jurassic (Sinemurian–Pliensbachian) before ending in the south in the Early–Middle Jurassic (Sinemurian–Aalenian/early Bajocian). Detrital zircons in sedimentary and volcaniclastic rocks were primarily derived from local erosion of pre-existing crustal terranes. Long-distance transport was likely for pre-Neoproterozoic and post-Cambrian Paleozoic zircons. Middle Jurassic detrital zircons in volcaniclastic rocks of the western Bahamas may have been deposited during ash-fall events from hotspot activity in the southeastern Bahamas.

Abstract The deposition of organic carbon–rich sediments during the Late Cretaceous in northern South America was controlled by global and local oceanographic, climatic, and tectonic variables. Key in establishing “source rock” depositional systems across the region were eustatic sea-level rise, warming global sea-surface temperatures, the formation of low-latitude saline bottom waters, and a relatively constant supply of fine-grained hemipelagic sediment (mostly derived from the south and east). Specific paleobathymetric conditions enhanced the development of stagnant water masses from the proto-Caribbean plate to Suriname. Organic-matter preservation was aided by the presence of these water masses across the region. Primary productivity was elevated above “normal” marine levels only in the protocentral Caribbean and along the ancestral Costa Rica/Panama island arc, or during seasonal upwelling in northern South America. Cooler, wetter climatic conditions that began in the late Santonian also were modified by regional and local variables. The development of new intermediate/bottom-water masses, increased polar heat transport caused by improved deep-ocean circulation, and fluctuations in volcanogenic CO 2 provided a background effect for local variables such as bathymetry and topography. The development of oxygenated high-latitude water masses provided a means for ventilation of stagnant, low-oxygen bottom waters across northern South America and the central Caribbean. Stronger seasonal upwelling (increased wind stress caused by better polar heat transport, and northward movement of the South American Plate into the zone of northward Ekman transport), more frequent fluvial outflow and deltaic deposition, and the submergence of key paleobathymetric barriers aided ventilation and subsequently diminished organic-matter preservation. Most of these variables had a positive impact on primary productivity and caused rapid changes in the diversity of planktonic foraminifera through the end of the Cretaceous.

ABSTRACT Well, seismic, and outcrop data were used to constrain the timing and development of unconformities associated with uplift events, and to aid in the definition of lithostratigraphic units of the Eocene of the southern Maracaibo and west-central Barinas/Apure basins. Palynology provided the main control on age dating of the sections, and graphic correlation was used to illustrate the amount of missing time in the lithostratigraphic record. An integration of biostratigraphic and lithostratigraphic data from wells and outcrops shows that deltaic to shallow marine conditions prevailed over most of the Maracaibo and west-central Barinas/Apure basins during the middle Eocene. Fluvial sedimentation dominated in areas of high standing basement features, and was accentuated by the development of incised valleys and their subsequent marine fill sequences along the southern and southeastern margins of the Maracaibo Basin. The main controls governing depositional facies, and the presence or absence of unconformities were differential structural motion and, to a lesser extent, relative sea level fluctuations. A basal Eocene to late Paleocene unconformity, as well as several intra-middle Eocene unconformities appear to have been primarily caused by a drop in base level (sea level + structural subsidence), forming incised valleys over most of the study area. Biostratigraphic (palynological) data, as well as field observations and subsurface mapping indicate that other unconformities within the upper middle Eocene, and between middle and upper Eocene strata are ravinement surfaces caused by transgressive marine re-working of pre-existing units. Sequence boundaries caused by incised valleys often overlie transgressive deposits, and can merge with ravinement surfaces and other sequence boundaries in areas of intense erosion. This is especially true for the multiple intra-middle Eocene unconformities and sequence boundaries observed and inferred for the eastern part of the Maracaibo Basin; these are not reflected in the outcrop belt to the south because the rate of accommodation space was not rapid enough to allow their preservation. Nevertheless, marine transgressive deposits in the southern part of the basin can be clearly recognized (where they are preserved) because of their contrast with otherwise dominant fluvio-deltaic strata; the recognition of specific transgressive events in the eastern part of the basin is less certain because they are less clearly differentiated within the mostly marginal marine strata of the area. The presence of favorable reservoir facies in Eocene rocks in the study area is primarily a function of original sandstone composition, grain size, and burial depth. In the northeastern part of the study area, estuarine channels and transgressive marine/tidal bars have the best reservoir properties in the Misoa Formation. In the southern Maracaibo Basin, fluvial channels and transgressive marine/tidal bars are also the best reservoirs in the Mirador Formation. Transgressive marine/tidal bars within the Carbonera Formation also have favorable reservoir characteristics. Productivity of the best Eocene reservoirs is generally high.

Abstract The morphology of platform margins as depicted on high-resolution seismic data and the development of outer shelf, eutrophic ecologic assemblages can be used as keys in understanding the evolution of carbonate platforms. The Baltimore Canyon platform, offshore United States East Coast, and the Liuhua platform, Pearl River Mouth Basin, People's Republic of China, exhibited the effects of environmental collapse prior to their extinction by drowning. Reprocessed seismic data, lithologic data, and biostratigraphic data from the Baltimore Canyon area show that environmental deterioration of the platform immediately preceded or was coincident with deltaic progradation. This implies that slope-front fill seaward of the platform is probably coeval with platform deposits, and that a previously identified carbonate sequence boundary may actually be an older drowning sequence. A seismic sequence boundary should be placed at the top of the youngest drowning sequence. Late-growth reefs appear to be discontinuous along both the Baltimore Canyon and Liuhua platform margins. The proximity of prograding deltas appears to be the main control on the location of late-growth reefs on both platforms, though tectonic subsidence (local faulting) may govern their distribution along the southern part of the Liuhua platform margin. The horizontal-planar onlap of basinal shales onto the Liuhua platform margin could be misinterpreted as always representative of an unconformable contact with the platform sequence. In reality, local differences in highstand off-bank transport of platform and platform margin sediments may have produced a progradational fore-slope (a wedge of fore-slope debris) in some areas, and only horizontal-planar onlap in others.

Abstract The Eastern Venezuela foreland basin is a structurally and stratigraphically complex foreland basin that contains the single largest oil accumulation in the world, as well as several other supergiant oil and gas fields. The development of these large hydrocarbon resources was a result of the widespread deposition of rich Cretaceous source rocks, multiple thick sandstone reservoirs, and long-term structural deformation. This paper reviews the general tectonostratigraphic development of the Eastern Venezuela foreland basin in light of recent petroleum exploration and production activities. This recent work, when combined with detailed stratigraphic data compiled during the past four decades, has helped to constrain and refine models of the geohistory of northeastern Venezuela. Data accumulated during nearly 100 years of oil exploration suggest that, despite the relative exploration maturity and drilling density in the basin, substantial undiscovered hydrocarbon resources may still be found along the present trend of giant fields.