ACRAMAN IMPACT HORIZON IN SOUTHWEST LAURENTIA: IMPLICATIONS FOR THE SUCCESSION OF GLOBAL ENVIRONMENTAL EVENTS ON THE EVE OF THE EDIACARAN RADIATION

We report high Ir concentrations in mid-Ediacaran (E) siliciclastic strata of SW Laurentia (SWL), associated with a regional oolite marker horizon ~2 m thick (“Johnnie oolite,” JO), enveloped in 20-30 m of laminated siltstone and fine sandstone of the lower Rainstorm Mbr of the Johnnie Fm. The two most Ir-rich samples are from the Kingston Range (KR), yielding c. 10 +/- 5 ppb Ir from a sample 0-1.5 cm above the JO, and 7 ppb in siltstones 2.5 m below it. About 80 km north in the Spring Mountains (SM), the upper boundary of the JO is dramatically ripped (boulder conglomerates, dm-scale rafting) and overlain by a few m of fine-grained sandstone, with a sample the near the base of the sand body yielding c. 1 ppb Ir. Medium-grained sandstones below the JO in the SM contain 5-10% medium to coarse albites and lithic grains that are internally deformed and angular in comparison with rounded host quartz grains. These textures and compositions are highly anomalous in SWL miogeoclinal sandstones, and are potentially consistent with an origin from Acraman target rock (the only known E impact event), which are now preserved both below the crater itself and in a widespread ejecta horizon in the lower Bunyeroo Fm. and correlatives. If correct, these observations (1) strengthen a temporal tie (based on the Johnnie-Wonoka CIE) between mid-E strata of Australia and one of the thickest E sections in Laurentia (c. 3000 m), and (2) indicate that the Rainstorm Mbr and its overlying incised valleys are a counterpart of the Bunyeroo/Wonoka interval and its incised valleys. It strains credulity to suppose that the singular, striking valley incisions in either of these thick sections are unrelated to the singular, peak expression of the Gaskiers glaciation. Thus, an overall sequence of: (1) Emerging glaciation, (2) onset of a deep Johnnie-Wonoka CIE (oxygenation?), (3) Acraman impact just before or during its early rapid descent, and (4) peak Gaskiers glaciation after the CIE recovery (580 Ma), would constitute the environmental prelude to the radiation. The pre-Gaskiers succession hints that the type Shuram CIE may be the second of a rapid “double pulse” of environmental stress in the global ocean between c. 585 and 575 Ma, just prior to the Avalonian radiation. This hypothesis would be falsified by the presence of Acraman ejecta at or near the base of the Shuram CIE.