A COMBINED CHEMOSTRATIGRAPHIC, LITHOSTRATIGRAPHIC AND SEQUENCE STRATIGRAPHIC APPROACH TO THE CHARACTERIZATION OF LOWER CRETACEOUS TO PALEOCENE FORMATIONS OF THE BROOKIAN SEQUENCE, ALASKAN NORTH SLOPE FOOTHILLS

The Cretaceous- to Paleocene- age sediments of the Brooks Range foothills of the Alaskan North Slope are an east- and northeast- prograding, non-marine to deepwater foreland basin clastic wedge represented lithostratigraphically in the study area by the Torok, Seabee, Canning, Schrader Bluff, Prince Creek, and Sagavanirktok formations. These units each contain proven hydrocarbon reservoirs along the Barrow Arch, and are considered frontier exploration targets in the foothills. Development of a reliable regional stratigraphy is hampered in this area by poor biostratigraphic resolution, local structural complexity, discontinuous outcrops and few well penetrations. In this study, we use chemostratigraphy, integrated with sequence stratigraphy and lithostratigraphy, to refine the stratigraphic framework for this frontier region.

Chemostratigraphic analysis was carried out on a total of 145 composited cuttings samples from four widely spaced exploration wells (Echooka 1, Susie 1, Lupine 1 and Aufeis 1), and a suite of 147 outcrop samples. Rock chips from each cuttings sample were carefully selected for analysis, using gamma ray data to determine the most representative lithology from each cutting interval. Whole-rock inorganic geochemical analyses were carried out on all samples using inductively coupled plasma - optical emission spectrometry (ICP-OES) and inductively coupled plasma - mass spectrometry (ICP-MS) analyses to quantify the concentrations of 50 elements.

Changes through in time in values of key element and element ratios, including but not restricted to, K₂O/Al₂O₃, K₂O/Rb, Cr/Al₂O₃ and Zr/Cr and U (ppm) enable geochemical differentiation of lithostratigraphic units, definition of finer scale stratigraphic units and integration of these units into a regional stratigraphic model. The geochemical variables used to characterize the formations reflect changes in sediment provenance, extrabasinal volcanism, changes in clay mineral species and paleo-redox conditions through time together with lateral facies changes.