A sequence stratigraphic framework for the Niobrara Formation in the Denver-Julesburg Basin
William Drake and Sarah Hawkins
The Upper Cretaceous Niobrara Formation in the Denver-Julesburg Basin records late Turonian to early Campanian climate and orbital cycles and tectonic events. The third-order transgressive-regressive events of the Niobrara Cyclothem govern the character of the pelagic and hemipelagic sedimentation of the formation. During sea-level highstands, the depositional system was dominated by increased biogenic productivity and lesser preservation of organic material. In contrast, lowstands involved increased terrigenous sediment influx and greater preservation of organic material. Mapping sequences that comprise these stratigraphic trends is important to operators currently exploiting the Niobrara petroleum system. We employ a sequence stratigraphic framework for the Niobrara Formation in the Denver-Julesburg Basin, based on log character but independent of lithostratigraphy, that reveals the architecture of key depositional packages. Based on detailed correlations using wire-line logs from over 2,000 wells, we present a series of isochore maps and cross sections that show: 1) stratigraphic thinning that we interpret as seafloor topographic features such as subtle submarine channel floors, 2) stratigraphic thickening that we suggest is compensational infilling of accommodation space, and 3) the chronology of abrupt shifts in the trends of these depositional features. We constrain the timing of tectonic/geomorphic features revealed in the isochore maps with published cyclostratigraphic analysis, biostratigraphy, and geochronology data. Findings include the following: 1) generally broad shelf deposition during the late Turonian was replaced in the Coniacian by subtle NW-SE- and E-W-oriented submarine channels, with compensational infilling by younger sequences; 2) the first evidence of paleobathymetric highs (e.g., the 'Wattenberg High'), and disruption of submarine channel orientations, appears during the middle Santonian (~85-84 Ma), and 3) upper Santonian-lower Campanian sequences are dominated by SW-NE-oriented architecture.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012