Sedimentology and paleogeographic reconstruction of the strata in and adjacent to the Sudbury impact layer in the northern Paleoproterozoic Animikie basin
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The Sudbury Impact Layer (S.I.L.) is dated at 1850 Ma and is located between the underlying Gunflint Formation, with an age of 1878±1 Ma that was obtained from zircons in a tuffaceous zone approximately 105 meters below the S.I.L., and the overlying Rove Formation that has an U-Pb zircon age of 1832 Ma which was obtained from tuffs 5-6 meters above the S.I.L. There is an 18 Ma hiatus between the Sudbury Impact Layer and from where the zircon was extracted from the overlying Rove Formation, and a 46 Ma hiatus between the Sudbury Impact Layer and the underlying Gunflint Formation. These large age anomalies associated with the hiatuses, along with little sedimentation between the tuffs that supplied the ages, suggest that periods of non-deposition and sub-aerial exposure eroded the land, resulting in a lack of sedimentation in the allotted age gaps. This time interval was investigated in a number of outcrops and cored drill-holes in the northern portion of the basin. Sedimentological aspects of the rocks were noted and samples collected for geochemical studies. The upper portion of the Gunflint Formation contains grainstones that were deposited in shallow water along with chemical sediments precipitated from Paleoproterozoic seawater. Positive Ce anomalies indicate oxygen production by stromatolites in the inter-tidal to very shallow sub-tidal lead to some oxygenation of the shallow nearshore. The chemical sediments in the limestone that overlies the Gunflint ankerite and chert had their calcite cement formed in meteoric phreatic conditions, with extremely elevated contents of vanadium and large negative cerium anomalies indicating these waters were significantly oxic. The overlying Sudbury Impact Layer shares these characteristic, though in the southeast it was probably deposited in a very wet, likely marine, setting. Ankeritic grainstones overlying the Sudbury Impact Layer refute the idea that the impact caused an end to iron formation deposition and show a transition from flooding and sub-tidal deposition to extensive sabkha development. The common occurrence of gypsum is indicated by the presence of its pseudomorphs forming bladed crystals, desert roses and vein systems. The Rove sea flooded over this surface after lithification.