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

Ediacarian sediments of the lower Ungoolya Group (similar to 580-565 Ma) in exploration-well Munta 1 of the Officer Basin of South Australia accumulat ed towards the base of a ramp on the southeastern flank of an axial foredee p, mostly in relatively deep water hemipelagic and turbiditic environments, and at palaeoequatorial latitude. Rapid subsidence at similar to 580 Ma cr eated 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 o f four successive elastic slope-aprons prograded northwestward across the M unta 1 locality. The individual relief of these four slope-aprons averaged at least 175 m. An epiclastic silt-dominated prograding frontal slope progr essively built up into warm surface waters above the thermo-/pycno-cline wh ere accretion of shoalwater carbonate culminated in peritidal platform depo sits and local evaporites. Renewed crustal subsidence and rapid marine tran sgression across the carbonate-platform caused the depositional system to f ounder, backstep southwestward on the structural ramp, and recommence progr adation of a new slope-apron. These depositional systems formed during a pr olonged period of arid climate that intensified in the period similar to 57 5-570 Ma, and possibly again near the end of the preserved lower Ungoolya G roup record in Munta 1. Chemostratigraphic excursions of delta(13)C(org) an d delta(13)C(carb) that occur at the stratigraphic base of each platform-ca rbonate at the top of the slope-aprons coincide with the bathymetric inters ection of the palaeo-pycnocline with the seafloor, suggesting that these ex cursions might at least partly result from an oceanic reservoir that was no t isotopically homogeneous and was partitioned for long periods of time at this physical boundary. Re-deposited Acraman Impact ejecta occurs throughou t 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 low er slope-apron and of mass-flow origin in the turbidites and debrites of th e upper slope-apron, and its presence above the 1810.5 m level in Munta 1 c onfirms an earlier prediction (based on comparative stratigraphy) as to the likely stratigraphic level at which the primary (but as yet undetected) Ac raman ejecta-layer should occur in Munta 1. The sediments contain well-pres erved microbial mats. They occur variously as: (1) autochthonous mars withi n epiclastic silts in the peritidal platform carbonates, and in lower slope -apron deposits where they probably grew in water depths of similar to 180- 200 m, probably below the lowermost reaches of the photic zone; (2) allocht honous structurally dismembered mats within debris-flow deposits of the upp er slope-apron that were sourced from upslope areas in the vicinity of the pycnocline; (3) tempestite deposits within the peritidal carbonates that co ntain large intraclasts of biolaminated siltstone; and (4) detrital fragmen ts and small intraclasts of mat-bound substrate that occur most abundantly within low-concentration turbidites and pelagites of both the upper and low er slope-apron. There are also abundant acritarchs within fine-grained turb idites, hemipelagites and marls. and in relatively deep-water carbonates. D isruption. fragmentation, dislodgement, and removal of autochthonous mat ma terial from the upper slope-apron in the vicinity of and above the pycnocli ne was accomplished by various mechanisms, both ambient and catastrophic. Episodic disturbance of the pycnocline by seiches was probably a major caus e of disruption and downslope removal and redeposition of mat material from the shallower parts of the photic zone on the upper slope-apron. Such dist urbance caused slumping Of in situ mats and generated slope-hugging debris- flows and low-concentration turbidity currents. These displaced abundant ma t fragments to lower parts of the slope-apron, and trapped interflows of su spension-load sediment containing abundant mat fragments at the pycnocline from where it spread laterally to generate long-continued rain of shallow-w ater mat kerogen to the distal slope-apron and deep-basin floor beyond. The kerogen of these mats occurs as either structurally discrete organic tissu e or as gel-like homogeneous sheets, both with and without discernible inte rnal structures. The organic tissue consists of filaments and dense cluster s of smooth spheres (similar to 1 mu m diameter) interpreted as fossil cocc oid microbes. Very finely crystalline carbonate, intimately associated with the coccoids, probably formed by microbial mediation in peritidal environm ents characterised by warm to elevated water temperatures. Biomarker distri butions of bitumen extracted from the Munta 1 microbial mats are characteri sed by abundant monomethyl and dimethyl alkanes of variable carbon-chain le ngth and isomer distributions. indicating a predominantly cyanobacterial in put, probably of diverse species populations at stratigraphically different levels. These cyanobacterial mars alternate with non-cyanobacterial microb ial mats that probably included anoxygenic photosynthetic green non-sulfur bacteria, with minor 'terminal anaerobic' sulfate-reducers and Archaea. Som e turbidite-siltstone samples. containing only allochthonous mat detritus, have a cyanobacterial biomarker signature that is strongly overprinted by o ther bacterial signals, including those of terminal consumers. Abundant alg ae (acritarchs) are present in many palynological preparations throughout t he section and show marked upward increase in taxonomic diversity. Sterane biomarker distributions are consistent with chlorophytes, rhodophytes, prym nesiophytes and/or dinoflagellates, or their precursors. The association of such diverse algal sterane biomarkers with abundant acanthomorph acritarch s suggests a close genetic relationship and demonstrates the great diversit y of planktonic algae during the terminal Proterozoic. 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