Caribbean Journal of Earth Science, 39 (2005).

 

 

Reflectance spectra of tropical vegetation as a response to metal enrichment in the substrate of west-central Puerto Rico

 

Johannes H. Schellekens, Fernando Gilbes, Garymar D. Rivera, Yuri C. Ysa, Samanta Chardón and Yolanda Fong

 

ABSTRACT. The northwest – southeast belt of Eocene rocks in west-central Puerto Rico contains a number of well-documented porphyry copper deposits. These deposits are characterized by felsic stocks and larger envelops of hydrothermally altered rocks that make these ideal targets for recognition from space-borne platforms. In the tropics, these deposits are usually hidden under thick soils and vegetation that complicate their recognition. The well-mapped geology of the belt and the well-documented mineral deposits allowed us to carry out an experimental study in which leaf reflectance of tropical vegetation growing on mineralized and barren substrates is compared. Leaves were collected from the top of canopies and the reflectance spectra between 400 and 800 nm were determined in the laboratory with a GER 1500 radiospectrometer. Spathodea campanulata (Tulipán africano) showed a reduction of reflectance around 550 nm and a shift towards larger wavelengths of the infrared plateau (‘redshift’).

 

 

UHP metamorphism in garnet peridotite, Cuaba unit, Rio San Juan Complex, Dominican Republic

 

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.

 

Hydrocarbon Charge Analysis of the SECC Block, Columbus Basin, Trinidad and Tobago

 

Steve Hertig and Mark Ver Hoeve

 

ABSTRACT. 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 Columbus Basin. Critical components to understanding hydrocarbon charge in any basin are 1) source rock distribution, 2) source rock maturation and 3) hydrocarbon migration paths. In the Columbus Basin, the hydrocarbon charge story is somewhat unique in that much of this very deep basin up to 12,000 m (40,000 ft) formed rapidly during Pliocene – Pleistocene time. The main driver of maturation occurred less than 5 Ma and was related to progradation of an ancestral Orinoco delta across the area. Maturation analysis suggests the source rocks pasted rapidly through the oil window during the early Pliocene and entered into the gas window in the late parts of the Pliocene - Recent to produce the mainly gas-prone hydrocarbon province seen today. Migration paths and trapping style are affected by structure and stratigraphy of post-Cretaceous rocks. The goal of this paper is to understand the hydrocarbon charge history for the 1.5 trillion cubic feet (TCF) so far discovered on the SECC Block.

 

 

Palaeocene to middle Eocene flysch-wildflysch deposits of the Caribbean area: a chronological compilation of literature reports, implications for tectonic history and recommendations for further investigation

 

Keith H. James

 

ABSTRACT. Literature reports upper Cretaceous to Middle Eocene flysch/wildflysch deposits from Mexico, Guatemala, Jamaica, Cuba, Puerto Rico, Barbados, Granada, Trinidad, Venezuela, Colombia, Ecuador and Peru. Early papers recording their widespread distribution and noting their tectonic implications are often overlooked by popular palaeogeographic and plate tectonic reconstructions of the Caribbean area. Detailed stratigraphic revision of some units has restricted their age of accumulation to the Palaeocene - Middle Eocene or to the K/T boundary event. Most of the deposits record violent interaction between the Caribbean and adjacent continental areas. They are classically attributed to a ‘Laramide (Incaic) Orogeny’. An alternative explanation is that they record one or more impacts of extraterrestrial matter. Units that have not received recent stratigraphic study should be revisited. A possible impact crater in the Venezuelan Basin requires investigation.

 

 

Arguments for and against the Pacific origin of the Caribbean Plate and arguments for an in situ origin

 

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 of Caribbean geology

 

Keith H. James

 

ABSTRACT. The complex area between the continental masses of North and South America is a collage of many continental, stretched continental, island arc and oceanic elements described by numerous works. Some areas are poorly exposed and not well known. Others are intensely explored and well documented. Syntheses of this geology popularly derive the Caribbean Plate from the Pacific and require major rotation of island arc elements and continental blocks along with major changes in plate migration direction. These models are complex and geometrically unlikely. This paper suggests a simple, in situ evolution from a Pangean configuration principally via regional (North ‑ South America), Jurassic-Late Cretaceous, WNW oriented sinistral transtension, followed by a Palaeocene–Middle Eocene compressional event and Oligocene-Recent, E‑W strike-slip between the Caribbean and American Plates.

 

 

The Humboldt Channel: Early Pleistocene extensional graben through eastern Venezuela and Trinidad

 

Oliver Macsotay

 

ABSTRACT. 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 Venezuela and northern Trinidad there is an E-W oriented depression, usually attributed to the El Pilar Fault system. This depression exposes thick sequences of siliciclastic sediments exposed in discontinuous outcrops. Their lithology includes conglomerates, sandstones and siltstones in massive beds, mostly of lenticular morphology. From west to east, the studied units are: Caiguire Formation, Chiguana Formation, Paria Formation (=Güiria Formation) and Talparo Formation (Compare and Caparo members). Some siltstones and thin bioclastic limestones contain molluscs, fish remains and some corals, bryozoans and barnacles. The molluscs belong to the gastropod Turritella maiquetiana zone and the bivalve Larkinia patricia zone, which indicate an Early Pleistocene age. The oligomictic faunal assemblage suggests high‑salinity, marine waters, with clay in suspension, locally stratified but well circulated at the surface. These deposits occur in a 345 km wide E-W outcrop zone, suggesting a short-lived channel open at both ends to marine waters, and called the Humboldt Channel, and represents part of Molassic cycle IV. These Early Pleistocene sediments, deposited during transtensional tectonics, were subjected to NW-SE transpression along the Cumana-El Pilar segment, which caused faulting and folding during Middle-Late Pleistocene. East of El Pilar, the tectonic inversion began with the start of subduction of the Gulf of Paria sediments beneath the Paria metamorphics in the Late Pleistocene, creating the present Gulf of Paria.

 

 

Palaeoclimatology of the Pleistocene-Holocene using marine molluscs and hermatypic corals from northern Venezuela

 

Oliver Macsotay and Raquel Caceres Hernandez

 

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 than living Caribbean equivalents: Astraea, Cerithium, Conus, Murex, Chicoreus, Oliva, Ancilla, Persicula, Turritella and Vasum. The bivalves are Crassostrea patagonica, Chama ainuosa bermudensis, Scapharca couvana, and these also suggest Atlantic temperate-affinity faunas dominating the coastlines, but not the offshore islands. The rainfall influence is noted in coastal areas in western and eastern Venezuela, while inland and the Llanos became arid.

The Holocene sediments are heterolithic and calcareous, with mangrove-forests on most of Venezuela’s coastline. Corals are mostly Porities and Millepora associations, with small sized Siderastrea colonies. Molluscs are very much like living assemblages, where invaders from West Africa appear briefly: Cerithium guinaicum, Cymatium spp. Climatic conditions are similar, but with higher coastal rainfall that contributed to the proliferation of mangroves. Early and Middle Pleistocene cooling of the surface waters in the southern Caribbean Sea preceded aridity of the Llanos and mountain glaciation of the Late Pleistocene (80 and 15 ka) of the Andean range.

 

Middle Eocene foreland sediments covered by late Oligocene foredeep turbidites on Margarita Island, northeastern Venezuela

 

Oliver Macsotay and Tulio Feraza

 

ABSTRACT. 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 Venezuela. Early Eocene fossils were collected in the upper part of the sequence.

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.

 

© Geological Society of Jamaica.