Terrigenous Beds Research Articles

Petrogenesis, geochemistry, and tectonic setting of a basaltic body within the Gercus Formation of northern Iraq: first record for Eocene anorogenic volcanic activity in the region

An abnormally odd and rare occurrence of a basaltic body has been discovered in the middle of the Gercus Formation within the large, double-plunging NW-SE trending Bekhair Anticline to the north of Duhok city, northern Iraq. This is the first discovery of such a volcanic body within the widely exposed Gercus Formation in northern Iraq, southeastern Turkey, and western Iran. This volcanic body, named by the authors the Gercus Basalt, has a long tabular/lenticular shape with a total length of ~4.5 km and a thickness ranging between 1 to 16 m. It is exposed on both limbs of the anticline and has a conformable relation with the terrigenous beds of the Gercus Formation. The basalt is greenish to grayish black in color, vesicular, amygdaloidal and porphyritic in texture, and consists of feldspar (Ab 85 An 2 Or 13 ), pyroxene (Wo 49 En 40 Fs 11 ), olivine (Fo 87.2 -Fa 12.6 -Tp 0.2 ), and their alteration products. Postemplacement hydrothermal fluids resulted in pervasive alteration of the Gercus Basalt and introduction of copper mineralization filling joints, fractures, and adjacent vesicles. The alteration processes are dominated by calcitization, zeolitization, serpentinization, chloritization, silicification, iddingsitization, and copper mineralization. Seventeen basalts have been analyzed for 63 major and trace elements including REEs and PGEs. The Gercus Basalt has very similar geochemical compositions to that of OIB. Excluding Cr, Co, Ni, and PGEs, it is enriched in all other analyzed major and trace elements relative to primitive mantle. The chondrite-normalized distribution of REEs shows a smooth pattern, high La/Yb (N) ratio of 23, and LREE enrichment relative to HREE, indicating alkaline magmatic origin. The studied rocks are basanites, alkaline basalts, melanephelinites, and picrites with an OIB-like anorogenic geochemical signature. The Gercus Basalt was derived from amphibole-bearing garnet-lherzolite subcontinental asthenospheric mantle, enriched with silicate metasomatism.

Diagenetic origin of ironstone crusts in the Lower Cenomanian Bahariya Formation, Bahariya Depression, Western Desert, Egypt

In this paper, a new interpretation of the ironstone crusts of the Bahariya Formation as late diagenetic products is provided. The siliciclastic Lower Cenomanian Bahariya Formation outcropping in the northern part of the Bahariya Depression (Western Desert, Egypt) is subdivided into three informal units that are mainly composed of thinly laminated siltstone, cross-bedded and massive sandstone, fossiliferous sandstone/sandy limestone and variegated shale. Abundant ironstone crusts occur preferentially within its lower and upper units but are absent in the middle unit. The ironstone crusts show selective replacement of carbonate components, including calcretes, by iron oxyhydroxides. More permeable parts of the terrigenous beds such as burrow traces, subaerial exposure surfaces, concretionary features and soft-sediment deformation structures led to heterogeneous distribution of the iron oxyhydroxides.A variety of diagenetic minerals, where goethite and hematite are the main end-products, were characterized by mineralogical analysis (XRD), petrography and SEM observation, and geochemical determinations (EMPA). Other diagenetic minerals include Fe-dolomite/ankerite, siderite, manganese minerals, barite, silica, illite/smectite mixed-layer, and bitumen. These minerals are interpreted to be formed in different diagenetic stages. Some minerals, especially those formed during eodiagenesis, show features indicative of biogenic activity. During burial, dolomite and ankerite replaced preferentially the depositional carbonates and infilled secondary porosity as well. Also during mesodiagenesis, the decomposition of organic matter resulted in the formation of bitumen and created reducing conditions favorable for the mobilization of iron-rich fluids in divalent stage. Telodiagenesis of the Cenomanian Bahariya deposits took place during the Turonian–Santonian uplift of the region. This resulted in partial or total dissolution of Fe-dolomite and ankerite which was concomitant to iron oxyhydroxide precipitation upon mixing with shallow oxygenated water.Circulation of reducing iron-rich fluids through fractures and inter and intrastratal discontinuities is proposed as an alternative model to explain the controversial source of iron for the ironstone crusts of the Bahariya Formation. The origin of iron-rich fluids is probably related to the basement rocks. The provided model relates the fluid movements through fractures and discontinuities with the preferential replacement of carbonates. This combination of processes is consistent with the heterogeneous geometries and the wide distribution of the ironstones.

Palaeogeography of the Upper Cretaceous-Eocene carbonate turbidites of the Northern Apennines from provenance studies

Abstract The Upper Cretaceous Helminthoid Flysch (HF) of the Northern Apennines consists of thick and regionally widespread deep-water carbonate turbidite successions, deposited during the initial stages of Alpine collision. The HF spans the time from Turonian to Early Eocene and is mainly composed of intrabasinal carbonate ooze mixed with clay; siliciclastic terrigenous beds are also present, but they are a volumetrically minor component of the successions. Petrographic and sedimentological signatures indicate that the HF was deposited in distinct basins located below the carbonate compensation depth. Bulk composition and heavy minerals of terrigenous beds indicate provenance from different crustal levels of the European and Adria plates. The petrographic and palaeobathymetric characteristics of these turbidites indicate the coexistence of an active-margin tectonic setting, a palaeogeographical position suitable for carbonate ooze production and storage, and limited supply of terrigenous detritus into the basin. Palaeotectonic reconstructions and stratigraphic data suggest that Adria represented a vast repository of penecon-temporaneous carbonate mud; the presumably intense seismic activity related to the pre-collisional Alpine orogeny promoted large-scale failures of shelf and/or slope biogenic muddy sediments, resulting in the deposition of a large volume of carbonate turbidites. Only occasionally, turbidity currents probably linked to exceptional fluvial floods generated pure terrigenous beds with different petrographic signatures for each HF succession.

Stratigraphy and Geochemical Characterization of the Middle Triassic - Carnian Sequence of the Lumiei Valley (Carnia, Northeastern Italy)

Stratigraphy of carbonate and terrigenous beds of the Anisian, Ladinian and Carnian of the Lumiei Valley (Middle-Western sector of Carnia, NE Italy) is updated and referred to the new chronological scale of Triassic. Western Carnian Alps were emerged in Early Anisian time. The sea transgressed southward and the region was covered by a thick sequence of lagoonal, clastic, and pelagic sediments. The sequence is thicker in this area, where a system of synsedimentary faults increased the subsidence. Tectonic activity increased during Middle(?) Anisian; this area turned into a topographic and structural high, divided by synsedimentary faults from neighbouring open-marine areas that received large amounts of terrigenous sediments. The latest Anisian and the Ladinian are characterized by a general progradation of the platform facies. Basin areas with pelagic and volcanic sediments were mostly developed in this area and towards the Carnian-Julian Alps. In the Late Ladinian a local resuming of volcanism is present in the transgression marked by the Acquatona Formation. The topmost part of the Ladinian is characterized by the general and wide diffusion of the inner platform, mostly with peritidal facies. At the Ladinian-Carnian boundary, tensional tectonics caused the breaking up of the Schlern platform and increased the subsidence. In the Early-Middle Carnian, this area is dominated by a marine transgression with marly limestone, pelagic, and terrigenous sediments. Moreover, major, minor and trace elements were determined. Statistical multivariate analysis methods were applied to experimental geochemical data to study the stratigraphic sequence. The purpose of this paper was to verify the capacity of these methods to describe the different lithologies of the sequence through its geochemical analysis. A multivariate statistical analysis based on geochemical parameters identifies distinct groups of sediments corresponding to lithological intervals of the examined sequence. In this way a good correspondence between geochemical groups and the lithologies was established.

Barbados: Architecture and implications for accretion

The island of Barbados exposes the crestal zone of the remarkably broad accretionary prism of the Lesser Antilles foreacrc. The architecture of Barbados is three‐tiered: an upper arched cap of Pleistocene reefs that record rapid and differential uplift of the island, an intermediate zone of nappes of mainly abyssal or deep bathyal pelagic rocks, and a basal complex whose lithotypes extend to substantial depth and may be representative of the bulk of the western or inner accretionary prism. The exposed basal complex consists of generally steeply dipping ENE to NE‐striking fault‐bounded packets which contain rocks of one of three lithic suites: terrigenous (quartzose turbidite and mudstone), debris flow, and hemipelagic (chiefly radiolarite). Present but imcomplete rock dating indicates that the terrigenous and hemipelagic suites and the pelagic rocks of the intermediate zone are age overlapping in Early and Middle Eocene time. Deformation within packets of the basal complex is systematic, pre‐ or synfault, and indicative of shortening that is generally normal to packet boundaries. A unit of terrigenous materials that probably underwent local resedimentation in the Miocene is recognized in wells, but its relationship to exposed rocks is uncertain. The packet‐bounding faults of the basal complex are interpreted to have been primary accretionary surfaces which may have been reactivated by later intraprism movements. Exposed sedimentary rocks of Barbados can be successfully assigned to contemporaneous depositional sites associated with an accretionary prism: terrigenous beds to a trench wedge that was connected to South American sediment sources, debris flow to trench floor or slope basin accumulations of material derived from the lower slope, hemipelagic to Atlantic plain strata, and pelagic rocks of the intermediate zone to deep outer forearc basin sites. The decollement at the base of the intermediate zone is probably due to uplift and arcward motion of the crestal zone of the accretionary prism with respect to the forearc basin during progressive prism growth. Principal uplift of the prism seems to have started, apparently abruptly, in the Miocene. Quaternary uplift of Barbados may be due partly to local diapirism. Paleogene subduction that created the arcward region of the prism probably occurred in a differently configured zone from the present one.

Mineral composition and accumulation conditions of the Riphean terrigenous beds in the Omolon Massif

(1981). Mineral composition and accumulation conditions of the Riphean terrigenous beds in the Omolon Massif. International Geology Review: Vol. 23, No. 12, pp. 1386-1390.

Paleoecology of some Carboniferous Pectinacea

Pectinacean bivalves in the Carboniferous of northern California, USA, occur in all observed biotic communities except bioherms of corals and productoid brachiopods. They show equal abundance in offshore, brachiopod-dominated habitats and nearshore, bivalve-dominated habitats, and are among the only common invertebrates in a ‘black shale’, restricted marine community. Different morphologic forms of the superfamily show little ecologic division. Pectinacea are least abundant in limestone, and most abundant in terrigenous beds, and bottom type was more important in controlling their distribution than water depth, current activity, or salinity.

Nearshore to offshore facies and depositional history of the Ordovician Daylesford Limestone New South Wales

Abstract The Daylesford Limestone is the basal formation of the Ordovician Bowan Park Group of central western New South Wales. The formation contains four main limestone types and minor intercalated terrigenous beds. Limestones are: (1) grain‐stone, (2) grey skeletal wackestone and packstone, (3) dark grey burrowed wackestone and packstone, and (4) dark grey burrowed lime mudstone. Grainstone and grey skeletal wackstone and packstone are dominant in eastern sections; they are laterally equivalent to, and interfinger with, dark grey muddy limestones that dominate western sections. Lithoclasts are abundant in the grainstone but are absent from muddy sections to the west except in thin beds above disconformities. The rock types of the Daylesford Limestone also tend to occur sequentially above some disconformities; the full sequence is: grainstone (or grey skeletal wackestone and packstone) grading up into dark grey burrowed wackestone and packstone and thence into dark grey burrowed lime mudstone. Each sequ...