Jeffrey T. Pietras

Strontium isotope record of paleohydrology and continental weathering, Eocene Green River Formation, Wyoming

Sr isotope ratios in carbonate-rich lacustrine strata provide highly resolved and geo- graphically specific records of past changes in weathering and regional drainage patterns. The Eocene Green River Formation is perhaps the best documented pre-Quaternary la- custrine unit in the world; therefore, these strata are ideally suited for studying the be- havior of Sr. The 87 Sr/ 86 Sr ratios measured for primary carbonate in lake expansion- contraction cycles of the Laney Member are directly linked to changing lake facies. Four distinct cycles are preserved in a 6.4 m interval of the Arco Washakie Basin No. 1 core, each represented by a vertical succession of transgressive stromatolite facies containing dolomicritic intraclasts, laminated micrite facies deposited during lake highstands, and lake-marginal dolomicrite facies, which represent subsequent lowstands. Initial 87 Sr/ 86 Sr ratios range from 0.711 99 to 0.713 31. The least radiogenic isotope compositions are as- sociated with laminated micrite that records increased regional runoff from Phanerozoic marine carbonate and Tertiary volcanic rocks during lacustrine highstands. The most radiogenic isotope compositions are associated with dolomicrite, recording a higher pro- portion of runoff from basin-bounding Precambrian uplifts and the exposed lake plain during lacustrine lowstand. The observed 87 Sr/ 86 Sr variations may also be due in part to changes in differential weathering of the surrounding landscape, where drier climates are associated with decreased differential weathering and, therefore, higher 87 Sr/ 86 Sr in runoff.

Lake basin response to tectonic drainage diversion: Eocene Green River Formation, Wyoming

A previously unidentified major sequence boundary within the Eocene Green River Formation separates fluctuating profundal facies of the Tipton Shale Member from evaporative facies of the Wilkins Peak Member. During deposition of the Tipton Shale Member, rivers entered the basin from the north, across the subdued Wind River Mountains, and deposited the southward prograding deltaic complex of the Farson Sandstone Member. Boulder-rich alluvial fan deposits overlie the Farson Sandstone adjacent to the Continental Fault, and correlate basinward to hypersaline lacustrine deposits of the Wilkins Peak Member. The alluvial fan deposits record a period of reverse motion on the Continental Fault and uplift of the southeastern Wind River Range, which diverted drainage away from the greater Green River Basin. This decreased inflow caused Lake Gosiute to shrink, exposing its bed to desiccation and erosion, and contributed to hydrologically-closed conditions and periodic evaporite deposition thereafter. This study is one of the first to demonstrate a direct relationship between movement along a specific basin-bounding structure, and a change in the overall style of lacustrine sedimentation. The identification of similar relationships elsewhere may challenge conventional interpretations of climate as the dominant factor influencing the character of lake deposits, and provide an important, but previously unexploited, approach to interpreting continental deformation and regional drainage organization.

Variable history of Quaternary ice-sheet advance across the Beaufort Sea margin, Arctic Ocean

The seismic stratigraphy and architecture of the Beaufort Sea shelf and slope are investigated using a comprehensive grid of high-resolution two-dimensional seismic reflection data. Three cross-shelf troughs, representing locations of former ice streams draining a 1000-km-long section of the Laurentide Ice Sheet (LIS) are examined: the Mackenzie, Amundsen Gulf, and M’Clure Strait systems. Dynamics of these paleo-ice streams influenced ice-sheet configuration and may have forced abrupt climatic change through delivery of ice and freshwater to the Arctic Ocean. A comprehensive understanding of their geometry and dynamics is crucial for constraining numerical models of the LIS. Evidence for two Quaternary ice advances to the shelf break is interpreted from the Mackenzie Trough. By contrast, seismic stratigraphy of the Amundsen Gulf Trough, 400 km east of the Mackenzie Trough, records at least nine Quaternary ice advances. Here, the outer shelf consists of stacked till sheets, extending to the shelf break and forming a trough-mouth fan. The contrasting glacial histories of these neighboring ice streams are explained by their positions within the LIS; the Mackenzie Trough ice stream was situated at the extreme northwest ice-sheet margin, whereas the Amundsen Gulf ice stream had a more central location and larger drainage basin, supplying significant quantities of ice and sediment to the Arctic Ocean through much of the Quaternary. The M’Clure Strait Trough probably possesses a similar architecture to the Amundsen Gulf Trough, and an even larger trough-mouth fan.

10 k.y. depositional cyclicity in the early Eocene: Stratigraphic and 40Ar/39Ar evidence from the lacustrine Green River Formation

Ar/ 39 Ar dating of sanidine from two interbedded tuffs reveals that the maximum average duration of depositional cycles in the Wilkins Peak Member, Green River For- mation, was ;10 k.y., marking the first time that subprecessional cycles have been rec- ognized in lacustrine strata. The origin of these cycles is uncertain, but may relate to a nonlinear climatic response to orbital forcing of insolation. Alternatively, regional tectonic and geomorphic controls on drainage stability may have promoted autocyclic delivery of sediment to the lake. Owing to an interaction between basin-floor relief and varying am- plitudes of lake expansion, only one-third of the cycles identified near the basin center are present near the basin margin. This spatial variability in the temporal completeness of the stratigraphic record is not apparent from examination of individual localities, indi- cating that studies based on time-series analysis from other lacustrine systems may need reevaluation.

Athabasca oil sands: Megatrap restoration and charge timing

The petroleum trap for the Athabasca oil sands has remained elusive because it was destroyed by flexural loading of the Western Canada Sedimentary Basin during the Late Cretaceous and Paleocene. The original trap extent is preserved because the oil was biodegraded to immobile bitumen as the trap was being charged during the Late Cretaceous. Using well and outcrop data, it is possible to reconstruct the Cretaceous overburden horizons beyond the limit of present-day erosion. Sequential restoration of the reconstructed horizons reveals a megatrap at the top of the Wabiskaw-McMurray reservoir in the Athabasca area at 84 Ma (late Santonian). The megatrap is a four-way anticline with dimensions 285 125 km (177 78 mi) and maximum amplitude of 60 m (197 ft). The southeastern margin of the anticline shows good conformance to the bitumen edge for 140 km (87 mi). To the northeast of the anticline, bitumen is present in a shallower trap domain in what is interpreted to be an onlap trap onto the Canadian Shield; leakage along the onlap edge is indicated by tarry bitumen outliers preserved in basement rocks farther to the northeast. Peripheral trap domains that lie below the paleospillpoint, in northern, southern, and southwestern Athabasca, and Wabasca, are interpreted to represent a late charge of oil that was trapped by bitumen already emplaced in the anticline and the northeastern onlap trap. This is consistent with kimberlite intrusions containing live bitumen, which indicate that the northern trap domain was charged not before 78 Ma. The trap restoration has been tested using bitumen-water contact well picks. The restored picks fall into groups that are consistent both with the trap domains determined from the top reservoir restoration and the conceptual charge model in which the four-way anticline was filled first, followed by the northeastern onlap trap, and then the peripheral trap domains.