Younger Facies Research Articles

The 1800-1770 Ma Colider silicic large igneous province in the Amazonian craton: crustal evolution using zircon in situ REE compositions, U-Pb ages and Hf isotope analyses

ABSTRACT This paper presents zircon U-Pb, REE and Hf data for Paleoproterozoic felsic volcanic rocks from the Colíder Group with outcrops in the southwestern Amazonian Craton. It is estimated that these rocks cover an area of 85,000 km2 with a volume of 2.7 × 106 km3. The Colíder volcanics are comprised of basalts, andesites and a variety of felsic rocks. The felsic rocks are characterized by rhyolite, andesite and dacite lavas with frequent intercalated pyroclastic and epiclastic deposits. Effusive and pyroclastic rocks are intruded in places by hypabyssal porphyritic rocks. The U-Pb ages obtained for the northeastern domain (1782–1776 Ma) are younger than the ages obtained for the southern domain (1800–1787 Ma). The NE volcanic facies show Hf TDM ages between 2.3 and 2.5 Ga (εHf from +1 to −3) and greater fractionation (Ce/Yb – 0.03–0.06) than the younger facies that has TDM from 2.4 to 2.7 Ga (εHf from −3 to −6) with Ce/Yb values of 0.025–0.030. The Hf isotope compositions, REE contents and U-Pb analyses suggest geographic (north-south) variations in composition, crystallization ages, and TDM age zonation during the generation of the Colíder large igneous province.

Seismogenic Deformation Structures in Microbialites and Mudstones, Silurian Lockport Dolomite, Northwestern Ohio, U.S.A.

Syndepositional deformation structures are present at two different stratigraphic levels in the Silurian Lockport Dolomite at a quarry in Maumee, Ohio. Such structures are absent in this unit at twelve other quarries in northwestern Ohio. The structures at Maumee, present within stromatolites, thrombolites, and variably bedded mudstones, include convolute layers, homogenization of bedding and breccia clasts, clasts of stromatolites, boudinage structures, normal, reverse, and thrust faults, and discordant and concordant breccias. The distribution of deformation structures within beds suggests substantial control from substrate rheology, possibly resulting from differences in degrees of lithification and from variations in water content. The extensive variety of syndepositional deformation structures, juxtaposed evidence for nearly synchronous compression and dilation, proximity (4.5 km) of the structures to the largest fault in northwestern Ohio (the Bowling Green fault), and the restricted geographic distribution of the deformation structures suggest seismic shaking as a probable mode of genesis. Similarities between features produced by seismogenic deformation of siliciclastic sediments and deformation features in the Lockport Dolomite provide additional evidence that the latter features are seismogenic. It is postulated that two earthquakes, one with a magnitude 5, caused the formation of the syndepositional deformation structures present in the youngest facies in the Lockport Dolomite at Maumee quarry. The Bowling Green fault, 4.5 km west of Maumee, was a growth fault during formation of the Lockport Dolomite, and is considered to have been the source of earthquakes that created the syndepositonal deformation structures in this unit at Maumee quarry. Evidence from the Lockport Dolomite in northwestern Ohio suggests that seismogenic deformation structures created by two earthquakes along the Bowling Green fault with magnitudes of 5 did not form in this unit over large areas (i.e., all of northwestern Ohio). This evidence challenges the suggestion that syndepositional seismogenic deformation structures are always formed over a large area.

Evidence for water storage and drainage at the base of the Laurentide ice sheet, south-central Alberta, Canada

AbstractGlaciogenic sediment up to 70 m thick is exposed in preglacial valley fills in south-central Alberta. The two youngest facies are comprised of glaciolacustrine sediments overlain by a subglacial till. Conformable relationships with the till and topographic relationships support a subglacial origin for the lake sediments. As these facies are restricted to the preglacial valley system, it is suggested that the preglacial valleys of south-central Alberta acted as natural cavities at the base of the Laurentide ice sheet. Sedimentology suggests these filled and drained often.

Regional Distribution and Chemical Characterization of Permian Capitan Fore-Reef Slope Dolomite: Implications for Paleohydrology: ABSTRACT

Dolomitized fore-reef slope facies recently have become significant targets for petroleum exploration in the Permian basin and elsewhere. Despite this, very little is known about the dolomitization process that largely controls porosity distribution in this facies. An integrated field, petrographic, and geochemical study has been conducted in the fore-reef slope strata of the Permian Capitan Formation to provide insight into this problem. Dolomitization of the Capitan fore-reef slope facies ranges from 0 to 100%. Regionally, the variation is related to stratigraphic age, with older forereef slope facies more dolomitized than younger facies. This generalization is consistent throughout the Guadalupe Mountains and appears to continue into the subsurface. On a local scale, more permeable beds were the first dolomitized, and within these, dolomitization decreases downslope. In the Capitan reef facies, dolomitization is restricted to vertical karst breccia pipes and other zones of high primary porosity. The majority of Capitan dolomite is finely crystalline (5-30 {mu}) with average {delta}{sup 18}O = 0.9 and {delta}{sup 13}C = 5.9. A second coarser (130-{mu}) dolomite generation is also present but is not as volumetrically important. The relative positive (for the Late Permian) {delta}{sup 18}O values indicates an evaporitively concentrated dolomitizing fluid. The most likely paleohydrologicmore » model for the main phase of forereef slope dolomitization calls for descending hypersaline brines passing through the reef facies in vertical karst breccia pipes and then following permeable beds down the foreslope. Several sources are possible for these brines, with the most likely being the hypersaline back-reef lagoonal environment.« less

Stratigraphy and Depositional Setting, Kingshill Limestone, Miocene, St. Croix, U.S. Virgin Islands

The Kingshill Limestone of St. Croix, U.S. Virgin Islands, was deposited during a transgressive-regressive cycle of the early and middle Miocene. Depositional bathymetry ranged from shallow lagoon and patch-reef environments to basin slopes, probably representing strandline to water about 500 m deep. Younger facies reflect a shoaling to probable subaerial exposure. Depositional processes included turbidity current movement of shallow-water sediments and benthic organisms into deep water, especially along the eastern margin of the depositional basin. Current indicators suggest that transport was largely toward the southwest or west. Sedimentation took place in a partly downfaulted basin that resulted from early Cenozoic vertical structural movements in the Caribbean. The basin now is the low-lying central plain of the otherwise mountainous island. Although the Kingshill has not been demonstrated to contain hydrocarbons, a relatively complete, albeit restored, stratigraphic section may aid in defining potential exploration targets in nearby areas of the Caribbean.

Sediments and geomorphology of the continental shelf off southern New England

The sediments and geomorphology of a portion of the northeastern continental shelf of the United States between Hudson Canyon and Georges Bank have been investigated. Wave-cut terraces at 13, 45, 65, and 80 fathoms indicate four former low sea levels, while concentrations of beach ridges around 35 fathoms reveal a fifth. The pre-Holocene drainage pattern, still exposed over part of the area, shows that Block Channel was the main stream of a system which drained most of southern New England. A large delta at the mouth of this channel near the edge of the shelf attests to the volume of sediments transported by the stream. The surface sediments on the western part of the shelf are Late Pleistocene fluvial deposits which were reworked by the Holocene transgression. Overlying these coarse deposits in the central part of the outer shelf is a sandy silt facies which is also relict. The youngest facies, and the only area of present deposition, is in the northeastern part of the shelf where sand which is eroded from Nantucket Shoals is being deposited on the shelf. An eastward directed coastal drift is indicated during certain lower sea level stages.