Late Devonian Extinctions: A Novel Conceptional Model for the Paleoenvironment of the Canning Basin, Gogo Formation, WA and Associated Oils: Exceptional Preservational Conditons of Biomarkers and Organic Matter in Carbonate Concretions and Basinal Muds

Abstract

One of the biggest mass extinctions in Earth's history took place in the Late Devonian. The most well know event occurred at the Frasnian – Famennian boundary, but there were also major biodiversity crises towards the end of the Givetian and Famennian time periods. Cause and nature (distinct events or several smaller pulses) of these extinctions remain under debate. Here we present a novel biomarker approach using methyltrimethyltridecylchromans (MTTCs) as indicators of freshwater incursions and possibly terrigenous input to a Late Givetian/Early Frasnian marine palaeoenvironment [1] also associated with the extinction of the Devonian reef systems. The abundance of gammacerane and Chlorobi biomarkers furthermore indicated persistent water-column stratification and prevailing but periodic photic zone euxinia (PZE). MTTCs are isoprenoid substituted aromatic compounds which are established palaeosalinity indicators [2]. Nevertheless, their source and formation pathway remain unknown. Our data would be consistent with an origin from early diagenetic condensation reactions of phytol with alkyl phenols (most likely higher plant derived) as it has been suggested previously by Li et al. [3]. The Gogo shales are remarkably immature and show the oldest report of original N contents. Here we also report the oldest occurrence of intact sterols in a Crustacean fossil [4] preserved for ca. 380 Ma within a Devonian concretion associated with the Gogo shales. The exceptional preservation of the organism is ascribed to microbially induced carbonate encapsulation involving persistent PZE (in contrast to oils that show evidence of periodic PZE due to thermal maturity [5]), preventing maximum decomposition and transformation so extending the occurrence of sterols in the geosphere by some 250 Ma. A suite of diagenetic transformation products of sterols were also identified in the concretion, demonstrating the remarkable coexistence of biomolecules and geomolecules in the same sample. We attribute the coexistence of steroids in a diagenetic continuum −extending from stenols to triaromatic steroids- to microbially mediated eogenetic reactions [6]. [1] Tulipani et al. (2013) Nature Scientific reports in prep. [2] Schwark et al. (1998), Org. Geochem. 29, 1921-1952. [3] Li et al. (1995) Org. Geochem. 23, 159-167. [4] Melendez et al. (2013) Geology 41, 123-126. [5] Maslen et al. (2010) Org. Geochem. 49, 1-17. [6] Melendez et al. (2013) Nature Scientific reports in press.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014