Reconnaissance sedimentology and hydrocarbon biomarkers of Ediacarian microbial mats and acritarchs, lower Ungoolya Group, Officer Basin

Ediacarian sediments of the lower Ungoolya Group (~580-565 Ma) in exploration-well Munta 1 of the Officer Basin of South Australia accumulated towards the base of a ramp on the southeastern flank of an axial foredeep, mostly in relatively deep water hemipelagic and turbiditic environments, and at palaeoequatorial latitude. Rapid subsidence at ~580 Ma created a marine basin of 200-300 m depth that was starved of sediment at the Munta 1 site until shortly after the Acraman Impact Event when the first of four successive clastic slope-aprons prograded northwestward across the Munta 1 locality. The individual relief of these four slope-aprons averaged at least 175 m. An epiclastic silt-dominated prograding frontal slope progressively built up into warm surface waters above the thermo-/pycno-cline where accretion of shoalwater carbonate culminated in peritidal platform deposits and local evaporites. Renewed crustal subsidence and rapid marine transgression across the carbonate-platform caused the depositional system to founder, backstep southwestward on the structural ramp, and recommence progradation of a new slope-apron. These depositional systems formed during a prolonged period of arid climate that intensified in the period ~575-570 Ma, and possibly again near the end of the preserved lower Ungoolya Group record in Munta 1. Chemostratigraphic excursions of δ¹³C(org) and δ¹³C(carb) that occur at the stratigraphic base of each platform-carbonate at the top of the slope-aprons coincide with the bathymetric intersection of the palaeo-pycnocline with the seafloor, suggesting that these excursions might at least partly result from an oceanic reservoir that was not isotopically homogeneous and was partitioned for long periods of time at this physical boundary. Re-deposited Acraman Impact ejecta occurs throughout at least 108 m of section in the sediments of the lowermost slope-apron. The reworked ejecta is of wind-borne origin in the hemipelagites of the lower slope-apron and of mass-flow origin in the turbidites and debrites of the upper slope-apron, and its presence above the 1810.5 m level in Munta 1 confirms an earlier prediction (based on comparative stratigraphy) as to the likely stratigraphic level at which the primary (but as yet undetected) Acraman ejecta-layer should occur in Munta 1. The sediments contain well-preserved microbial mats. They occur variously as: (1) autochthonous mats within epiclastic silts in the peritidal platform carbonates, and in lower slope-apron deposits where they probably grew in water depths of ~180-200 m, probably below the lowermost reaches of the photic zone; (2) allochthonous structurally dismembered mats within debris-flow deposits of the upper slope-apron that were sourced from upslope areas in the vicinity of the pycnocline; (3) tempesite deposits within the peritidal carbonates that contain large intraclasts of biolaminated siltstone; and (4) detrital fragments and small intraclasts of mat-bound substrate that occur most abundantly within low-concentration turbidites and pelagites of both the upper and lower slope-apron. There are also abundant acritarchs within fine-grained turbidites, hemipelagites and marls, and in relatively deep-water carbonates. The association of such diverse algal sterane biomarkers with abundant acanthomorph acritarchs suggests a close genetic relationship and demonstrates the great diversity of planktonic algae during the terminal Proterozoic. Crown copyright (C) 2000 Published by Elsevier Science B.V. All rights reserved.