Organic matter and palaeoenvironmental signals during the Early Triassic biotic recovery: The Salt Range and Surghar Range records

Abstract

Latest Permian to the Middle Triassic mixed siliciclastic–carbonate shelf deposits of the northern Gondwana margin have been studied in four sections (Nammal, Chhidru, Chitta–Landu, and Narmia) in the Salt Range and Surghar Range of Pakistan. Sedimentological and palynofacies patterns combined with a high resolution ammonoid based age control have been used to assess environmental changes such as sea-level change, distance from the shore, and oxygenation conditions of the sections in the aftermath of the end-Permian mass extinction. The base and the top of the Early Triassic are marked by second order sequence boundaries (SRT1, SRT8). Within the Early Triassic two third order sequence boundaries could be delineated by means of palynofacies analysis and sedimentology, one near the Dienerian–Smithian (SRT2) and the second one near the Smithian–Spathian boundary (SRT5). The extinction event at the Smithian–Spathian boundary seems to be closely associated to the latter globally recorded sea-level low stand. Five additional sequences of undetermined order (SRT3, SRT 4, SRT5/1, SRT6, and SRT7) are reflected in the sedimentological record of the studied sections. The observed changes in the composition of the particulate organic matter (POM) indicate a general shallowing upward trend, which is modulated by smaller transgressive–regressive cycles supporting the sedimentologically defined sequences. The POM is mostly dominated by terrestrial phytoclasts and sporomorphs. The strongest marine signal is reflected by increased abundance of amorphous organic matter (AOM) in the lower part of the Ceratite Marls at Nammal (late Dienerian) and Chhidru (earliest Smithian) and the Lower Ceratite Limestone at Chitta–Landu (late Dienerian). AOM of marine origin is characteristic for deeper, distal basinal settings and is preferentially preserved under dysoxic and anoxic conditions, indicating reduced oxygen conditions during these intervals. Up-section transgressive events are reflected by increased numbers of acritarchs, reaching up to 50% of the POM. Well oxygenated conditions and low total organic carbon contents (TOC) continue up to the top of the Early Triassic (Mianwali Formation). The most pronounced terrestrial influx is expressed in the Middle Triassic. Organic carbon isotope data parallel the carbonate carbon isotope records from the Tethyan realm; therefore, they reflect real global changes in the carbon cycle independent of the OM composition. The biomarker study of the apolar hydrocarbons of three samples from the Nammal section indicates an enhanced bacterial productivity, especially in the Smithian and Spathian, reflected in high relative abundances of hopanes. POM, TOC data and redox sensitive biomarkers together with high resolution biostratigraphy demonstrate that well-oxygenated environmental conditions prevailed in the Early Triassic with the exception of the Dienerian to earliest Smithian interval. The POM assemblages of Late Permian to late Griesbachian age indicate well oxygenated conditions during this time interval. There is no evidence in support of an anoxic event in the late Griesbachian in these sections.