Petrography and geochemistry of siliciclastic rocks of the Middle Eocene Gercus Formation, northern Iraq: Implications for provenance and tectonic setting

Abstract

Red coloured Middle Eocene Gercus Formation is a widely exposed rock unit in northern Iraq, southeastern Turkey, and northwestern Iran. Siliciclastic rocks (mudstones and sandstones) are the predominant rocks of this formation which were studied petrographically and geochemically. The Gercus Formation also contains a few beds of limestone, gypsum, and conglomerate. The sandstones are of two types, red and white, mostly immature litharenites, consisting of lithics (85%), quartz (12%), and feldspars (3%). The rock fragments consist dominantly of angular chert and rounded limestone and smaller amounts of sedimentary, magmatic, volcanic, and metamorphic lithics indicating proximity of their source rocks. The provenance discrimination diagrams of the detrital grain counts in sandstones show recycled and collision orogen sources. Petrographic, heavy minerals and major and immobile trace element (e.g. La, Th, Sc, Zr, Hf, and Ti) discrimination diagrams indicated that the Gercus Formation was originated under a collision tectonic setting above a subduction zone as part of an Oceanic Island Arc (OIA), where its sediments were derived from mixed sources, including recycled sediments of chert-and carbonate-rich formations and igneous-metamorphic ophiolitic complexes of north to north-east Iraq and adjacent Iranian and Turkish counterparts. The results also indicated a mixed marine (lacustrine and deltaic) and non-marine (fluviatile) origin for the Gercus Formation. The red sandstones, white sandstones, and mudstones show minor differences in their major, trace, and rare-earth element (REE) contents and have generally similar Post Archean Australian Shale (PAAS)-, Upper Continental Crust (UCC)-, and CI-chondrite-normalized distribution patterns indicating their common origin. The average of elements in the siliciclastic rocks is depleted in the high-field-strength elements (HFSE), the large-ion lithophile elements (LILE), and REE; and enriched in some rock-forming elements (RFE) and mantle rock-forming elements (MRFE) (Mg, Ca, Cr, Co, Ni) and depleted in others (Si, Al, Fe, Cu, Zn, Ga) relative to PAAS and UCC; whereas, they are depleted in Fe, Mn, Mg, Co, Ni, Cu, Zn, Ga, Na, and P, and enriched in all other RFE, MRFE, LILE, HFSE, and REE compared to CI-Chondrite. The REE content of these rocks is very low (23-34 ppm) and their LREEs are enriched six times relative to HREEs. These geochemical characteristics suggest a provenance dominated by basic/ultrabasic rocks. The U/Th, V/Cr ratios and the authigenic U values show an oxic depositional environment for the studied siliciclastic rocks.

Provenance of sediments from Barra del Tordo and Tesoro beaches, Tamaulipas State, northwestern Gulf of Mexico

“…Furthermore, the relative chondrite-normalized REE pattern and the Eu anomaly have also been used to infer the source of clastic sediments (e.g. Cullers 2000;Hernández-Hinojosa et al 2018;Kettanah et al 2021;Madhavaraju et al 2021). The REE patterns of the Tordo and Tesoro beach sediments are both with a significantly negative Eu anomaly, indicating felsic igneous rocks as their sources (Fig.…”

Detrital zircon U–Pb geochronology and geochemistry of the Riachuelos and Palma Sola beach sediments, Veracruz State, Gulf of Mexico: a new insight on palaeoenvironment

“…Therefore, the combined results obtained from the Pearson's Correlation Coefficient and the Principal Component Analysis indicate the association of major and trace elements with detrital materials rather than biogenic components. Harnois 1988) have been widely used to infer intensity of weathering (e.g., Ota et al 2017;Anaya-Gregorio et al 2018;Raza and Mondal 2018;Tawfik et al 2018;Hossain et al 2018;Kettanah et al 2020). However, a few authors (Cullers 2000;Buggle et al 2011;Garzanti et al 2015) documented that sediments are affected by various factors like sorting, recycling, and having significant proportions of carbonate or phosphate minerals.…”

“…1 Map showing the geology and geography of Riachuelos and Palma Sola beach areas, Gulf of Mexico (Source: Carta Geológica; Scale: 1:1,000,000; modified after Anaya-Gregorio et al 2018). L. = Lagoon; R. = River literature (Basu et al 2016;Carranza-Edwards et al 2019;Casse et al 2019;Karudu 2019;Men et al 2019;Kettanah et al 2020). Although clastic sediments are affected by factors such as weathering, diagenesis, and mineral fractionation during transport, their mineralogical-geochemical composition is mainly related to the parent rock types and the influence factors of the source (McLennan et al 1993;Basu 2017;Ramos-Vázquez et al 2017Bansal et al 2018;Ndjigui et al 2019;Rivera-Gómez et al 2020).…”

Provenance of northern Gondwana Lower Ordovician sandstones (Khabour Formation, northern Iraq) revealed by detrital zircon using LA‐ICP‐MS dating

“…providing clues to a possible tectonic setting in which the sediments were formed (Basu, Bickford, & Deasy, 2016;Folk, 1980;Kettanah, Armstrong-Altrin, & Mohammad, 2020;Tortosa, Palomares, & Arribas, 1991).…”