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 t...

Amelioration of marine environments at the Smithian–Spathian boundary, Early Triassic

“…The SSB also marked a major change in global climate, with strong tropical sea-surface cooling (Sun et al, 2012;Romano et al, 2013) and a steepening of the latitudinal temperature gradient (Galfetti et al, 2007). To date, however, the SSB event has received detailed study only in several sections in South China (Galfetti et al, 2007;Liang et al, 2011) and the Salt Range of Pakistan (Hermann et al, 2011). Here, we report the SSB event from a new Lower Triassic section in southern Guizhou Province, South China.…”

“…Lycopsid-dominated assemblages were replaced by conifer-dominated or mixed lycopsid-conifer vegetation, as indicated by palynological data from Pakistan (Hermann et al, 2011), Norway (Galfetti et al, 2007;, and central Europe (Kurscher and Herngreen, 2010). A similar floral change was reported from the SpathianAnisian boundary in Hungary (Looy et al, 1999), suggesting some variation in the timing of terrestrial floral recovery in different regions of the world.…”

Vegetation history across the Permian–Triassic boundary in Pakistan (Amb section, Salt Range)

“…The contact between the Chhidru Formation and the overlying Mianwali Formation represents an erosional unconformity interpreted as a sequence boundary (Mertmann, 2003;Hermann et al, 2011b) and representing a temporal hiatus between deposition of the two formations. The extinction of marine biota has been described to coincide with the formational boundary between the two formations (Schindewolf, 1954).…”

“…The present study deals with the basal part of the Mianwali Formation, including the Kathwai Member and the basal part of the Mittiwali Member, namely the Lower Ceratite Limestone and the lowermost interval of the Ceratite Marls (e.g. Waagen, 1895;Kummel and Teichert, 1970;Guex, 1978;Hermann et al, 2011b).…”

“…Hermann et al, 2011b;Brühwiler et al, 2012). At Nammal, the Pakistani-Japanese Research Group (1985) divided the Kathwai Member into three units and placed the Permian-Triassic boundary in the middle unit, whereas Wardlaw and Mei (1999) documented the occurrence of H. parvus near the base of the Kathwai Member in the Salt Range area (without mentioning the exact locality); in some successions the lowermost unit of the Kathwai Member is not preserved (Mertmann, 2003).…”