sedimentology and paleogeographic reconstruction of the strata in and adjacent to the sudbury impact layer in the northern paleoproterozoic animikie basin
abstract
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.