MC - MAP COORDINATES: LAT: N011300; N011300; LONG: W0772200; W0772200.
IL - ILLUSTRATION: illus. incl. sketch map.
RF - REFERENCE SOURCE: GeoRef, Copyright 2000, American Geological Institute.
IS - ISSN: 1056-148X
CO - CODEN: #03223
AN - ACCESSION NUMBER: 1999-017825
UD - UPDATE CODE: 199907
Registro 4888 de 5614 - GeoRef Disc 4: 1993-1996
BK - BOOK TITLE: Foraminifera sensing of paleoceanographic changes across the Cenomanian-Turonian (C-T) boundary interval in Colombia and Pueblo, Colorado.
BA - BOOK AUTHORS: Arango-Claudia
CP - COUNTRY OF PUBLICATION: United-States
PY - PUBLICATION YEAR: 1995
DG - DEGREE GRANTED: Master's
DI - DEGREE GRANTING INSTITUTION: University of Colorado. Boulder, CO, United States. Pages: 159. 1995.
LA - LANGUAGE: English
AB - ABSTRACT: Events that occurred during deposition of the Cenomanian-Turonian (C-T) boundary interval strongly impacted the composition and structure of planktic foraminiferal assemblages. The C-T boundary interval in Colombian and Pueblo sections can each be divided into at least three distinct planktic foraminifera assemblage groups on the basis of relative abundance, diversity, and numerical dominance relationships between species. The stratigraphic distribution of these groups correlates with the delta (super 13) C excursion record of the gradual establishment of the oceanic anoxic event, its intense phase, and its gradual dissipation. Early Late Cenomanian assemblages suggest a population in equilibrium in a relatively stable oceanic paleoenvironment, where species of several morphotypes and several habitats within the water column were partitioning resources. The recovery of benthic foraminifers indicates somewhat higher oxygen levels in comparison with the upper-Upper Cenomanian interval where only benthic foraminifers tolerant to low-oxygen conditions were found. During the mid Late Cenomanian, with the establishment of the oceanic anoxic event, the oxygen-minimum zone may have moved upward in the water-column causing the extinction of deep-water Rotalipora and then the disappearance of intermediate-water Praeglobotruncana, thus changing the planktic assemblage. The extinction of 4/15 species of planktic foraminifera is registered during this time. During the uppermost Cenomanian, at the intense phase of the OAE (highest values of delta (super 13) C), diversity of planktic foraminifera decreases and assemblages were dominated by shallow-dwelling Heterohelix. Heterohelicidae started to diversify, increase in size and change in shape in specimens of Heterohelix moremani led to a defined and persistent new species, Heterohelix globulosa. The first common occurrence of Heterohelix globulosa can be used to identify the Cenomanian-Turonian boundary. The Early Turonian recovery of planktic foraminifer diversity indicates dissipation of the OAE with the waning and contraction of the oxygen-minimum zone.
AB - ABSTRACT: The eruption of the Nevado del Ruiz (Colombia) on 13 November 1985 produced lahar flows, resulting in extensive deposits. The imbrications of flat clasts within the deposits as well as the orientations of large boulders resting on the surface of these deposits were measured. The imbrications of the long (A) axes of the small clasts were found to dip 25 degrees upstream against the lahar-flow direction (with a dispersion of 30 degrees ) indicating transport of such clasts by normal stream flow. Contrary to this, the boulders (mostly on the surface of the deposits) are aligned with their long (A) axes normally to the lahar flow, indicating rolling on the surface. Both orienting mechanisms occurred in runoff pulses subsequent to the main event which produced only unordered deposits "en masse". Thus, the genesis of the emplacement of the lahar deposits in question has been mechanically explained.
RF - REFERENCE SOURCE: GeoRef, Copyright 2000, American Geological Institute.
IS - ISSN: 0364-6017
CO - CODEN: XICIA5
RN - REPORT NUMBER: C 1103-A
AN - ACCESSION NUMBER: 1994-015059
UD - UPDATE CODE: 199407
Registro 4893 de 5614 - GeoRef Disc 4: 1993-1996
TI - TITLE: The Guerrero suspect terrane (western Mexico) and coeval arc terranes (the Greater Antilles and the Western Cordillera of Colombia); a late Mesozoic intra-oceanic arc accreted to cratonal America during the Cretaceous.
AB - ABSTRACT: The Guerrero suspect terrane, composed of Late Jurassic-Early Cretaceous sequences, extends from Baja California to Acapulco and is considered to be coeval with the late Mesozoic igneous and sedimentary arc sequences of the Greater Antilles, the West Indies, Venezuela and the Western Cordillera of Colombia. These sequences represent the remnants of an arc which accreted to the North American and northern South American cratons at the end of the Cretaceous. In western Mexico, the arc sequences built on continental crust consist of high-K calc-alkaline basalts, andesites and rhyolites enriched in LREE with abundant siliceous pyroclastic rocks interbedded either with Aptian-Albian reefal limestones or red beds. They do not show magmatic changes during the arc development. In contrast, the arc sequences built on oceanic crust show an evolution with time. Arc activity began with the development of depleted low K-tholeiitic mafic suite (Guanajuato igneous sequence), followed first by mature tholeiitic basalts and then by calc-alkaline olivine basalts interbedded with micritic limestones and radiolarian oozes of Early Cretaceous age. At the end of the arc growth, during Aptian-Albian times, calc-alkaline pillow basalts and andesites poured out in the volcanic front while shoshonitic olivine basalts extruded in the back arc. The tholeiitic and shoshonitic mafic rocks as well as the calc-alkaline lavas are mildly enriched in LREE, Y and Nb and show high epsilon (sub Nd) ratios, typical of oceanic arcs. In contrast, the calc-alkaline mafic suite enriched in LREE, Y and Nb exhibits lower epsilon (sub Nd) ratios suggesting that it was derived by the partial melting of a mantle source contaminated either by Paleozoic subducted sediments or old source enrichments (OIB). The Cretaceous arc rocks of the Greater Antilles, interbedded with and/or capped by Aptian-Albian limestones, the Cretaceous andesites of northern Colombia, the Cretaceous tholeiitic and calc-alkaline volcanic rocks of Venezuela, and the Cretaceous volcano-plutonic arc assemblage of Tobago share a similar magmatic evolution with the western Mexican oceanic arc. The tholeiitic plutono-volcanic assemblage of Tobago, depleted in LREE and characterized by high epsilon (sub Nd) values is similar to the Guanajuato volcano-plutonic sequence of Mexico, considered to represent the pristine stage of the arc. The mature tholeiitic sequences exposed in the proto-Caribbean arc show flat to moderately enriched LREE patterns like those of the Guerrero terrane. However, felsic plutonic and volcanic rocks prevail in the Caribbean. Calc-alkaline suites, accompanied locally by shoshonitic lavas, characterize the end of arc magmatic activity in both places. Thus, the geochemical features of the Late Jurassic-Cretaceous arc series of the Guerrero terrane and the proto-Caribbean are consistent with the following plate tectonic model. The Guerrero terrane and the proto-Caribbean probably belonged to the same intra-paleo-Pacific arc system the development of which was related to the subduction of oceanic basins fringing the North and northern South American cratons. This subduction zone was WSW dipping. While subduction was going on, these magmatic arcs drifted, moved closer to the North and South American cratons, and finally collided with the American borderlands at different periods during the Cretaceous. The late Mesozoic Guerrero and proto-Caribbean arc sequences show striking similarities with the Miocene calc-alkaline lavas dredged from the Banda Ridges, the North Marianas Seamount Province, and the Halmahera and Philippine arcs. We suggest that the diverse but mostly submarine segments of this late Mesozoic intra-Pacific arc rimmed the North and South American cratons as much as these Tertiary arcs rim Southeast Asia.