Abstract

The petrographic, geochemical, and isotopic studies were carried out on the carbonate rocks of Early Cretaceous (upper part of Chia Gara, Sarmord, and Qamchuqa formations) in Northeastern Iraq to understand the source of the rare earth elements (REE), the effect of the terrigenous input, and the inference of the depositional and post-depositional imprints, redox condition and oceanic anoxic event (OAE). Three sections were chosen (Barsarin, Ranya, and Zewe) which are located in different tectonic zones (Imbricated, the boundary between High Folded and Imbricated, and High Folded Zones, respectively). One hundred-four samples were examined petrographically and for δ¹³C and δ¹⁸O isotopes, 53 samples were selected for major, trace, and rare earth elements analysis. A petrographical study revealed five distinct facies for upper part of Chia Gara, the most prevailed were wackestone and packstone; six facies for Sarmord, the most common is mudstone lithofacies; and seven facies for Qamchuqa, the most common types are packstone and grainstone. Numerous diagenetic processes, including neomorphism, dissolution, cementation, compaction, and fractures, have an impact on the limestones of the upper part Chia Gara Formation. Cementation, neomorphism, dissolution, compaction, and dolomitization all affect Sarmord. Nonetheless, paleo-karsting, tectonic fracture, dissolution, cementation, and mechanical/ chemical compaction are the primary diagenetic characteristics of the carbonate of the Qamchuqa. The carbonate of Chia Gara was deposited in the deep environment, while Sarmord carbonates under the shelf environment, and Qamchuqa in shallow to open marine, including the inner ramp.

The PAAS-normalized REE patterns for the upper part of Chia Gara and Sarmord carbonates are characterized by MREE bulge, whereas the Qamchuqa samples exhibit a seawater-like REE pattern, with LREE depletion relative to HREE. The average Y/Ho ratios for the upper Chia Gara, Sarmord, and Qamchuqa carbonates are 32.68, 32.37, and 45.42, respectively, from which the latter is consistent with the superchondritic ratio observed in seawater, in contrast, the upper part of Chia Gara and Sarmord formations are lower. The La/Sc, La/Co, Th/Sc, Th/Co, Cr/Th ratios, and the La-Th-Sc ternary plot, additionally the Th/Co-La/Sc bivariate, imply the terrigenous sediments in the studied carbonates originated from felsic source rocks, with some input of mafic sources for Qamchuqa carbonate. Most of the analyzed samples of Sarmord and Qamchuqa show negative Ce anomalies (Ce/Ce* = 0.93, 0.54) and relatively positive Eu anomalies (Eu/Eu* = 1.15, 1.05). Positive Eu anomalies might be caused by the contamination of seawater-like signatures by detrital materials. The variations in ΣREE content are related to the amount of associated terrigenous materials and scavenging processes by iron oxides/hydroxides. Fluctuation of oxic-dysoxic and anoxic conditions are recognized by the Mn*, Th/U ratios, authigenic U, negative Ce anomalies, Fe/Al, V/Cr, and Ni/Co ratios. The typical C-values, Sr/Cu, and Rb/Sr ratios, indicate the Early Cretaceous carbonates were deposited under arid and hot climate.

The δ¹⁸O versus δ¹³C bivariate diagram showed that most of the studied carbonates are plotted in the field of marine limestone, with some samples of the upper part of Chia Gara and Sarmord carbonates plotted in the fields of warm water skeleton and Qamchuqa carbonate in burial cement. The Z values indicate the limestone type is of marine origin, and the paleosalinity of ocean water at the depositional time is 31‰. The paleotemperatures for carbonate precipitation are 30.4, 30.3, and 27.3ºC for upper part of Chia Gara, Sarmord, and Qamchuqa, respectively. The geochemical parameters (Mn/Sr ratio and relationship between δ¹³C and δ¹⁸O) demonstrate the studied carbonates preserved the seawater isotopic signature. Several positive δ¹³C excursions have been documented globally during the Cretaceous period, influenced by a series of oceanic anoxic events (OAE). The Early Aptian (Selli, 1a) Event was detected in this study.