Abstract The Raton basin of southern Colorado is geologically analogous to other Rocky Mountain Laramide basins that contain areally and volumetrically large accumulations of natural gas reservoired in tight Cretaceous and Tertiary sandstones and located in the deeper parts of the basins. Such basin-centered gas accumulations are recognized as a class distinct from conventional structural or stratigraphic traps. Based on geologic analogy, specific detailed geologic mapping, observed gas shows, and bore-hole log analysis, a basin-centered gas accumulation is postulated to exist in the deeper part of the Raton basin in the Trinidad Sandstone.

Abstract Current models for the tectonic evolution of northeastern South America invoke a Palaeogene phase of inter-American convergence, followed by diachronous dextral oblique collision with the Caribbean Plate, becoming strongly transcurrent in the Late Miocene. Heavy mineral analysis of Cretaceous to Pleistocene rocks from eastern Venezuela, Barbados and Trinidad allow us to define six primary clastic domains, refine our palaeogeographic maps, and relate them to distinct stages of tectonic development: (1) Cretaceous passive margin of northern South America; (2) Palaeogene clastics related to the dynamics of the Proto-Caribbean Inversion Zone before collision with the Caribbean Plate; (3) Late Eocene–Oligocene southward-transgressive clastic sediments fringing the Caribbean foredeep during initial collision; (4) Oligocene–Middle Miocene axial fill of the Caribbean foredeep; (5) Late Eocene–Middle Miocene northern proximal sedimentary fringe of the Caribbean thrustfront; and (6) Late Miocene–Recent deltaic sediments flowing parallel to the orogen during its post-collisional, mainly transcurrent stage. Domain 1–3 sediments are highly mature, comprising primary Guayana Shield-derived sediment or recycled sediment of shield origin eroded from regional Palaeogene unconformities. In Trinidad, palinspastic restoration of Neogene deformation indicates that facies changes once interpreted as north to south are in fact west to east, reflecting progradation from the Maturín Basin into central Trinidad across the NW–SE trending Bohordal marginal offset, distorted by about 70 km of dextral shear through Trinidad. There is no mineralogical indication of a northern or northwestern erosional sediment source until Oligocene onset of Domain 4 sedimentation. Paleocene–Middle Eocene rocks of the Scotland Formation sandstones in Barbados do show an immature orogenic signature, in contrast to Venezuela–Trinidad Domain 2 sediments, this requires: (1) at least a bathymetric difference, if not a tectonic barrier, between them; and (2) that the Barbados deep-water depocentre was within turbidite transport distance of the Early Palaeogene orogenic source areas of western Venezuela and/or Colombia. Domains 4–6 (from Late Oligocene) show a strong direct or recycled influence of Caribbean Orogen igneous and metamorphic terranes in addition to substantial input from the shield areas to the south. The delay in the appearance of common Caribbean detritus in the east, relative to the Paleocene and Eocene appearance of Caribbean-influenced sands in the west, reflects the diachronous, eastward migration of Caribbean foredeep subsidence and sedimentation as a response to eastward-younging collision of the Caribbean Plate and the South American margin. Supplementary material: Location maps and detailed heavy mineral data tables are available at http://www.geolsoc.org.uk/SUP18365.

Abstract The Pliocene B4 sandstone is an important gas-bearing reservoir in the Oil-bird field, Columbus Basin, southeast Trinidad. The B4 sandstone is dominated by fine-grained, massive, and parallel-laminated sandstones inter-bedded with thinly laminated siltstone and very fine-grained sandstones deposited in a shelfal setting. A detailed sedimentologic study of the B4 reservoir was performed by integrating 60 ft (18.3 m) of core, 3770 ft (1149 m) of borehole image data, open-hole logs, mud logs, and biostratigraphic data. A total of 12 sedimentary facies were described and interpreted on the basis of sedimentation processes using a genetically oriented facies analysis approach. Six facies associations were identified based on the gamma ray (GR) pattern and the vertical facies association. Core data show mostly massive sandstones with very little to no bioturbation. These massive sandstones commonly alternate with intervals having diffuse lamination, which appear as a recurrent feature within the massive bodies. The tran-sitional recurrence of massive and parallel-laminated sandstones indicates velocity fluctuating and sustained turbulent flows. Sedimentologic evidences suggest that the origin of the B4 sandstone could be related to the paleo-Orinoco River-related product of turbidity (hyperpycnal) outflows that extended into the Columbus Basin. The associated occurrence of plant debris favors the interpretation of a direct fluvial supply by rivers in flood stage (hyperpycnal systems). The lateral correlation of facies associations throughout seven wells allowed the identification of six depositional units, named from base to top, A, B, C, D, E, and F. Facies maps for the B4 reservoir have been developed and were constrained by paleocurrent data extracted from image data. The B4 reservoir sand shows a progradational pattern reflecting the infill of a fault-controlled depocenter. Paleoflow data indicate axial transport roughly parallel to the main fault system, indicating that faults were active and controlled the accommodation space. The proposed hyperpycnal depositional model will introduce substantial changes for the prediction of the geometry and position of sandstone accumulations. The hy-perpycnal model predicts the occurrence of sand accumulations in the lower parts of the paleolandscape, whereas the higher parts of the basin (margins) are characterized by fine-grained sediments. Thus, this new depositional model could represent a drastic shift in the prospectivity guide for new exploration plays.

Abstract The heavy mineral compositions of sandstones in Trinidad and Barbados record the onset of Andean-related erosion and a reduction of craton-derived sediments into NE South America. The changing provenance was deduced by comparing heavy mineral assemblages interpreted from ancient sandstones with associations recognized in modern sands that can be reasonably correlated to existing tectonic domains. The impact of the Andean orogeny across the margin was to introduce a suite of minerals characteristic of low-temperature metamorphism that today is prevalent adjacent to the Caribbean Mountain belt and differs from the zircon-rich assemblage produced within cratonic plains. Twenty-one Paleocene–Late Pliocene sandstone samples from Trinidad revealed systematic changes in mineral diversity and maturity that recorded this provenance transition, and suggests Andean erosion during deposition of the Late Oligocene Nariva Formation. Similar to Palaeogene sandstones of Trinidad, four Eocene Scotland Formation samples from Barbados support craton derivation, but with additional evidence of minor Andean input probably due to the proximity of the Scotland Formation delivery systems to an earlier uplift episode. By the Late Miocene, most of the sediments delivered into Trinidad basins were supplied from the Andean orogeny as suggested by the relative abundance of minerals of this affinity. The heavy mineral records of Trinidad and Barbados are similar to that described across northern South America from both modern and ancient environments that collectively mark the uplift of the Andean mountain belt, with its strong influence on drainage patterns and reservoir provenance along this sector of the continental margin. Supplementary material: Sample location coordinates, sample and outcrop photographs, and summary outcrop sections are available at http://www.geolsoc.org.uk/SUP18728 .

ABSTRACT The volcanic stratigraphy of the central Gulf of California margin of the Baja California peninsula preserves a valuable record of the transition from subduction of the Farallon plate (24–12 Ma) to oblique rifting (<12 Ma). Although strike-slip faults (as well as normal faults) are common in oblique rifts and are abundant on the new (younger than 6 Ma) seafloor in the Gulf of California, none has been previously reported in the onshore central Baja California margin. This study focused on a previously unmapped region in the central Baja California margin near Mulegé, where we identified a strike-slip fault, termed the Potrero fault, and described the regional magmatic and structural context for this fault. We did this by using geologic mapping of volcanic-volcaniclastic lithofacies, supported by petrography, geochemistry, and 40Ar/39Ar geochronology. The Potrero fault is a vertical fault that strikes N10°W, with dextral-oblique (down-to-the-east) slip. This fault juxtaposes older rocks on the west with younger rocks on the east. The older rocks on the west side of the Potrero fault are assigned to the Middle Comondú Group, which is early Miocene in age. They consist largely of a >800 m red bed sequence of coarse-grained andesitic volcanic debris-flow deposits (proximal facies) that transition westward into fluvial conglomerates and sandstones (distal facies). The proximal facies has interstratified coarse-grained trachyandesite block-and-ash-flow tuffs with a 40Ar/39Ar age of 18.72 ± 0.24 Ma. This section is cut by mafic- to intermediate-composition dikes, with lesser plugs, that have 40Ar/39Ar ages of 16.88 ± 0.30 Ma to 14.85 ± 0.05 Ma. This early Miocene assemblage is truncated by an angular unconformity and overlain by Pliocene high-Sr/Y trachyandesite lavas, with an 40Ar/39Ar age of 4.02 ± 0.04 Ma. The younger rocks on the east side of the Potrero fault are assigned to the Upper Comondú Group, which is middle to late Miocene in age. This unit is dominated by small lava shields, with diameters of 2–9 km and thicknesses up to 300 m. The lava shields have basaltic andesite, basaltic trachyandesite, andesite, high-Sr/Y trachyandesite, and dacite compositions, with 40Ar/39Ar ages of 13.39 ± 0.03 Ma to 10.74 ± 0.08 Ma (four samples). At two localities in the map area, the Upper Comondú Group lava shields rest in angular unconformity on the Middle Comondú Group red beds and dikes, and the eruptive equivalents of the dikes are missing along this unconformity. We correlated this unconformity with the unconformity at the top of the Middle Comondú Group on the west side of the Potrero fault to estimate a vertical component of slip of at least 800 m down-to-the-east across the Potrero fault. The lateral component of slip is not known, because the regions to the north and south are unmapped, so piercing points cannot be identified. The Middle Comondú Group in the Mulegé–La Trinidad area forms part of a regionally extensive, early Miocene lithostratigraphic unit, hundreds of meters thick, that outcrops for a distance of 500 km along the central to southern Gulf of California margin. It thickens and coarsens eastward, through what is now the Concepción Peninsula, where it also contains early Miocene dikes and hypabyssal intrusions and is similarly capped by an angular unconformity, with eruptive equivalents of the dikes and intrusions missing along the unconformity. We propose that the laterally extensive Middle Comondú Group was deposited in a rift basin, bounded by a west-dipping normal fault system that lay to the east of what is now the Concepción Peninsula, in the present-day offshore Gulf of California. We infer that the thick, coarse-grained volcanic andesitic debris-flow deposits of the Middle Comondú Group were shed from large andesite arc stratovolcanoes (Comondú arc) that also lay to the east in the present-day Gulf of California. We interpret the volumetrically minor block-and-ash-flow tuffs, dikes, and hypabyssal intrusions of the Middle Comondú Group to record minor magmatism in a forearc position. We also suggest that the angular unconformity at the top of the Middle Comondú Group records thermal uplift that occurred as the arc axis swept westward (trenchward) into the region, at ca. 14 Ma, due to continued slab rollback that began in the Oligocene under western Mexico. The Upper Comondú Group lavas in the Mulegé–La Trinidad area form part of a Middle to Upper Miocene lithostratigraphic unit, hundreds of meters thick, which outcrops for a distance of 700 km along the central to southern Gulf of California margin. This unit consists largely of andesite and basaltic andesite lavas erupted from stratovolcanoes in the axis of the Comondú arc. The Upper Comondú Group lavas thicken dramatically eastward toward the Bahía Concepción fault, a down-to-the-west normal fault that bounds the Concepción Peninsula on its west side. We thus infer that this fault became active in middle Miocene time (14 Ma). This fault records westward encroachment of normal faulting concurrent with the westward sweep of the arc axis, from the Gulf of California into Baja California. The Pliocene high-Sr/Y basaltic trachyandesite lavas that form the structurally highest part of the Mulegé–La Trinidad area form an erosional escarpment that does not extend to the Potrero fault, so the lavas cannot be used to determine whether the Potrero fault was active before, during, or after their eruption. The Pliocene high-Sr/Y basaltic trachyandesite lavas are a previously unidentified part of the regional postsubduction suite of “bajaites,” attributed to slab window magmatism.

Abstract We performed zircon fission-track dating on twenty low-grade metamorphic rocks from the Northern Range and eight unmetamorphosed sandstones from the Central Range of Trinidad. Reset and partially reset zircon populations from the Northern Range gave a consistent fission-track age of 11.7 ± 1.1 Ma. This dates the cooling of the Northern Range through 240 ± 25°C. Assuming a geothermal gradient of 25°C km 1 which is unproved, and a constant rate of uplift, the Northern Range uplifted at an average rate of 0.7 to 1.0 mm yr 1 from the Late Miocene to Recent. If the uplift was not at a constant rate, then the true uplift rates may have been greater than 1.0 mm yr -1 . In areas of lower grade metamorphism, Northern Range zircon fission-track ages were not reset, and thus gave information on the provenance of the Cretaceous sediments. Permian and Late Jurassic fission-track ages determined for the Maastrichtian Galera Formation sandstones are interpreted to have been derived, respectively, from granitoids and rift-related volcanics along the northern South American margin. Albian and Early Silurian/Late Ordovician zircon fission-track ages from the Lower Cretaceous Toco Formation have no provenance local to Trinidad but may have been derived further west, possibly from the Caribbean island arc and northern Andes respectively. This difference between the two formations is entirely consistent with structural and stratigraphic work which show the Toco Formation to be part of the Sans Souci Group, an allochthonous terrane transported from the west with the Caribbean plate. The Galera Formation is part of the Northern Range Group, which makes up the majority of the Northern Range, and is inteipreted to have been deposited on the northern South American margin, probably within 400 km of its present location. Zircon fission-track ages from unmetamorphosed sandstones of central Trinidad’s Pointe-a-Pierre Formation constrain deposition of the formation to later than 34 Ma (Lower-Middle Oligocene or later). This is much younger than the faunally estimated Lower Eocene (55 Ma) depositional age, probably because the fauna, which are “arenaceous,” have been reworked. Similar zircon fission-track ages are found in central Trinidad’s Nariva Formation, which is faunally dated as Middle Oligocene to Middle Miocene age.