of tropical vegetation as a response to metal enrichment in the substrate of
Johannes H. Schellekens, Fernando Gilbes, Garymar D. Rivera, Yuri C. Ysa, Samanta Chardón and Yolanda Fong
The northwest – southeast belt of Eocene rocks in west-central
UHP metamorphism in
garnet peridotite, Cuaba unit, Rio San Juan Complex,
Richard N. Abbott, Jr., Grenville Draper and Shantanu Keshav
ABSTRACT. Spinel-bearing garnet peridotite (olivine + diopside + enstatite? + garnet + spinel + late magnesiohornblende + late serpentine) and a corundum-bearing variant are associated with hornblende gneiss and schist (hornblende + plagioclase + quartz + rutile +/- garnet +/- biotite +/- epidote) and retrograded eclogite (hornblende + symplectic diopside-plagioclase + garnet + quartz +/- epidote) in the Cuaba amphibolite unit of the Cretaceous Rio San Juan complex. The occurrence is unusual because the garnet peridotite was educted at an ocean-ocean convergent plate boundary.
Four attributes suggest low-P/T (UHT) conditions: (1) Sequence of mineral assemblages, (2) nature of associated rocks, (3) Cr-in-clinopyroxene, Mg2Si2O6-in-clinopyroxene thermobarometry (>1.8 Gpa, >900 ˚C), and (4) tectonic setting. However, the thermobarometry is questionable because of low Cr in clinopyroxene. On the other hand, CFMAS equilibria involving components in olivine, garnet, clinopyroxene, spinel, and corundum suggest pressures from 2.8 Gpa (810 °C) to 3.5 Gpa (740 °C). These high-P/T (UHP) conditions are at odds with other indications (1, 2, 4, above) for low-P/T conditions. The contradiction highlights the unusual character of the rocks, but may be consistent with emplacement and uplift at an ocean-ocean convergent plate boundary.
Elsewhere, Alpine-type garnet peridotite is associated with what had been deep subducted continental rock, and ascent was presumably driven by the buoyancy of the continental material. In the present case, the buoyant force is not obvious. Indications of initially very high temperature at a modest depth in the mantle (~55 km) argue for the involvement of hot upwelling mantle. We offer alternative models wherein the garnet peridotite originates (1) in the mantle wedge above the subduction zone or (2) in the mantle beneath the subduction zone. In the first instance, the process may be related to extension in the forearc. In the second instance, the process may be related to subduction of an ocean ridge or mantle plume.
Analysis of the SECC Block,
Steve Hertig and Mark Ver Hoeve
Rocks of the Cretaceous age Naparima Hill and Gautier formations are well
established as the principal source rocks for the gas/condensate fields of the
Palaeocene to middle
Eocene flysch-wildflysch deposits of the
Keith H. James
Literature reports upper Cretaceous to Middle Eocene flysch/wildflysch deposits
Arguments for and
against the Pacific origin of the
Keith H. James
ABSTRACT. This paper discusses arguments presented in support of the Pacific model of Caribbean Plate origin. They do not stand up to close scrutiny. The paper continues with a series of arguments, supported by known geology, for the in situ origin of the plate.
A simple synthesis
Keith H. James
The complex area between the continental masses of North and
The Humboldt Channel: Early Pleistocene extensional graben through eastern Venezuela and Trinidad
Vertical crustal movements associated with convergent and transpressional plate
boundaries commonly produce small, short-lived basins which are rapidly filled
by large thicknesses of sediment. In eastern
Palaeoclimatology of the Pleistocene-Holocene using marine molluscs and hermatypic corals from northern Venezuela
Oliver Macsotay and Raquel
ABSTRACT. Along the Venezuelan coastline and its islands, 47 outcrops of marine sediments were studied. Their ages, determined by radiometric and palaeontological means, ranged from Middle Pleistocene to Holocene. A total of 234 taxa was identified: Coelenterata 30; Gastropoda, 116 and Bivalvia 88. The Middle Pleistocene sediments are iron-rich sands and clays, only locally with bioclastic limestones. There is barely any coral fauna and the molluscs are small-sized. The assemblages are bivalve-dominated with Atlantic affinities: Crassostrea patagonica, Ostrea puelchana, Anomalocardia spp., Gemma gemma and Macoma venezuelana. Gastropods include Cerithidea pliculosa and Melongena margaritana also suggesting temperate-affinity faunas, strongly influenced by heavy rainfalls on coasts and islands.
Late Pleistocene sediments are more calcareous allowing the development
of reefal systems with medium biodiversity of corals: Pocillopora elegans, Acropora
cervicornis and Milleaster sp.
The molluscan assemblages are gastropod-dominated, with smaller sized shells
The Holocene sediments are heterolithic and calcareous, with
mangrove-forests on most of
Middle Eocene foreland sediments covered by late Oligocene foredeep turbidites on Margarita Island, northeastern Venezuela
Oliver Macsotay and Tulio Feraza
On eastern Margarita lsland local outcrops are found of thick marine, Paleogene
sediments deposited on Late Cretaceous-Early Paleocene pelagic sequence of
phlyllite-chert- limestone alternations. The eastern outcrops are a typical
flysch sequence (Pampatar Formation), more than 1,000 m thick, with a basal
sequence of reworked andesitic rocks, radiolarian limestones of Campanian age
and cherts of Maastrichtian age. These are lithologically identical to the
Matatere Formation of the Lara Nappe of western
The western outcrops were deposited in a subsiding trough: Las Bermudez diamictites, whose massive mudstones exhibit submarine canyon-fill lenses containg conglomerates and diamictites up to 800 m thick. At the base, lenses of limestone/sandstone (Los Bagres) alterations carry molluscs of early Middle Eocene age. The Los Bagres is interpreted as slide deposits of shelf sediments; the Las Bermudez, as slope deposits, identical to the Guaiquera Formation from the Villa de Cura Nappe’s sedimentary cover. Conformably above, El Datil shales are distal turbidites deposited during the late Middle Eocene in a foreland.
Thrust on or slid over the former, the Punta Mosquito Formation’s kilppe, is a succession of limestones, calcisiltites and lutites (calcareous turbidites) of the same age as the Datil Shale. Unconformably above, massive siltstones and shales of the Caracolito Formaton contain greywake sandstones, slumps and trace fossils belonging to the Nereites ichnofacies indicating a deep basinal location. Preliminary palaeontological examination suggests a late Oligocene age.