KIMBERLITES, HOST ROCKS, HIATUSES AND PALYNOLOGY; NORTH-CENTRAL ALBERTA KIMBERLITE FIELDS, CANADA

Palynological and thermal maturity studies of Albian through Early Paleocene clasts from kimberlites from the Lac de Gras area (Slave Craton, 65O N, NWT) resulted in inferences on the timing of hiatuses, and the thickness and depositional environments of the now eroded sedimentary cover. Coincidences between the projected hiatuses and cycles of kimberlite eruptions were recognized. These interpretations were made from miospore and dinoflagellate assemblages and associated organic debris recovered from rock fragments mixed into the kimberlite during eruptions and subsequent fall-back of rim deposits into the crater.

Current studies involve the Buffalo Head Hills and Birch Mountain kimberlite fields of north-central Alberta, 57.5 O N, where, in contrast to the Lac de Gras area, a Cretaceous host rock cover still exists. The palynological study of about 400 core samples, from a continuous Cretaceous bedrock section spanning vertically over 560 m, is part of a collaborative project that includes sedimentology, volcanology, palynology, geochemistry and borehole geophysical studies on kimberlite and its associated sedimentary host rocks. The objectives include determining chronostratigraphic correlations between sedimentary basin configurations and kimberlite emplacement.

Preserved host rocks range in age from Albian to Late Campanian. Palynological assemblages indicate two distinctive events: coeval Turonian to Santonian volcanism-sedimentation and a younger eruptive event identified by the presence of Maastrichtian and Paleocene miospores in sedimentary xenoliths derived from now eroded strata. Maximum transgressions occur in the Albian/Early Cenomanian and Santonian. Times of dominantly terrestrial sedimentation include the Middle Cenomanian and Late Campanian and younger sediments. A hiatus with a maximum extent of Early Turonian to Early Santonian spans major kimberlite emplacement events and includes the time of deposition of the otherwise wide-spread transgressive Second White Specks Formation. Within a setting of normally flat lying strata, vertical shifts in the elevations of lithologic markers and palynological datums suggests kimberlite emplacement was associated with local structural bulges and depressions, possibly related to extensional tectonics.