Abstract Uranium-lead (U-Pb) geochronology studies commonly employ the law of detrital zircon: A sedimentary rock cannot be older than its youngest zircon. This premise permits maximum depositional ages (MDAs) to be applied in chronostratigraphy, but geochronologic dates are complicated by uncertainty. We conducted laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) of detrital zircon in forearc strata of southern Alaska (USA) to assess the accuracy of several MDA approaches. Six samples from Middle–Upper Jurassic units are generally replete with youthful zircon and underwent three rounds of analysis: (1) LA-ICP-MS of ∼115 grains, with one date per zircon; (2) LA-ICP-MS of the ∼15 youngest grains identified in round 1, acquiring two additional dates per zircon; and (3) CA-TIMS of the ∼5 youngest grains identified by LA-ICP-MS. The youngest single-grain LA-ICP-MS dates are all younger than—and rarely overlap at 2σ uncertainty with—the CA-TIMS MDAs. The youngest kernel density estimation modes are typically several million years older than the CA-TIMS MDAs. Weighted means of round 1 dates that define the youngest statistical populations yield the best coincidence with CA-TIMS MDAs. CA-TIMS dating of the youngest zircon identified by LA-ICP-MS is indispensable for critical MDA applications, eliminating laser-induced matrix effects, mitigating and evaluating Pb loss, and resolving complexities of interpreting lower-precision, normally distributed LA-ICP-MS dates. Finally, numerous CA-TIMS MDAs in this study are younger than Bathonian(?)–Callovian and Oxfordian faunal correlations suggest, highlighting the need for additional radioisotopic constraints—including CA-TIMS MDAs—for the Middle–Late Jurassic geologic time scale.
The Alaska Division of Geological & Geophysical Surveys, in collaboration with the Alaska Division of Oil & Gas, is engaged in a multi-year analysis of Cook Inlet basin that began in May 2006.This program is being implemented in two phases.The focus of Phase I is the stratigraphic trap potential of Tertiary strata in upper Cook Inlet; the focus of Phase II is the reservoir and source rock potential of middle Jurassic through Maastrichtian rocks of the Peninsular terrane.The chapters in the volume represent preliminary results from fi eldwork conducted in September 2006 and May 2007 on the Kenai Peninsula.Chapter A by LePain and others presents a facies analysis of the Tyonek, Beluga, and Sterling Formations in exposures on the Kenai Lowland and between Seldovia and Coal Cove (Port Graham).This report includes detailed measured stratigraphic sections of these formations.Lateral facies changes are documented in incised valley-fi ll deposits of the Tyonek Formation between Barabara Point and Coal Cove.In this area the Tyonek Formation is limited to small paleovalleys that were eroded into older metamorphic rocks of an emergent accretionary wedge (McHugh Complex., Chugach terrane).Lateral facies changes in bluff exposures of the Beluga and Sterling Formations are also documented and used to highlight contrasting sand body geometries in these units.Includes Appendix A, pages 33-97.Chapter B by Finzel and others presents a more detailed facies analysis of the Tyonek Formation occupying a paleovalley cut into the Chugach terrane between Seldovia and Barabara points.This work documents a signifi cant change in sediment composition up-section in the valley fi ll; near the base of the valley fi ll, sediment composition refl ects derivation from local sources in the underlying Chugach terrane, whereas higher in the valley fi ll, sediment composition and textures suggest derivation from more regional sources and transport in a large drainage network.The latter part of the valley fi ll appears to represent the transition from valley-confi ned sedimentation to axial fl uvial depositional systems located basinward of the incised valley network.Chapter C by Blodgett and others presents a brief description of a one-day fi eld trip to exposures of Mesozoic strata in Port Graham and at Point Naskowhak.Preliminary fossil identifi cations are presented, along with their age signifi cance.Well-preserved and lightly abraded bivalves in sandstones that include features suggestive of rapid deposition in relatively deep water indicate short transport paths between the shallow water source area for the fauna and the depositional site.Chapter D by Loveland presents mercury injection capillary pressure data generated by George Bolger at PetroTech Associates, Houston, TX, from outcrop samples collected from the Beluga and Sterling Formations.Nearly all samples are tied to measured stratigraphic sections, which provide facies context for the data.Most of our measured sections are presented in Appendix A of Chapter A (LePain and others, this volume).The sample number includes the measured section number and the stratigraphic position of the sample in the measured section.For example, sample 07JRM005-16.5 was collected 16.5 m above the base of measured section 07JRM005 (fi g.A11, Appendix A, LePain and others, this volume).
We report a likely neoceratopsian manus track from an exposure of the Nanushuk Formation along the Colville River in northern Alaska. The track described here containts the impressions of five digits, arranged as an arc, which identify this specimen as a manus. Details of the impression suggest that it is neoceratopsian rather than ankylosaurian. The length of the chord of the arc of the track is approximately 25 cm, which is half the size of manus tracks found west of Denver, Colorado, USA attributed to the 10 m long Maastrichtian Triceratops. The Nanushuk Formation is a succession of complexly intertonguing marine and nonmarine strata interpreted as shelf, deltaic, strandplain, fluvial, and alluvial overbank deposits. Deposited in the foreland basin north f the Brooks Range, the rock unit is present throughout most of the northern foothills belt and subsurface of the central and western North Slope coastal plain. Fossil and radiometric data place outcrop within the Albian. If the identification of the track is correct, this is one of the earlies occurrences of neoceratopsians from North America. The occurrence of this track in Alaska substantiates the biogeographic model of faunal exchange between Asia and North America through a Cretaceous land bridge known as Beringia.
Abstract The Lower Cretaceous Fortress Mountain Formation occupies a spatial and temporal niche between syntectonic deposits at the Brooks Range orogenic front and post‐tectonic strata in the Colville foreland basin. The formation includes basin‐floor fan, marine‐slope and fan‐delta facies that define a clinoform depositional profile. Texture and composition of clasts in the formation suggest progressive burial of a tectonic wedge‐front that included older turbidites and mélange. These new interpretations, based entirely on outcrop study, suggest that the Fortress Mountain Formation spans the boundary between orogenic wedge and foredeep, with proximal strata onlapping the tectonic wedge‐front and distal strata downlapping the floor of the foreland basin. Our reconstruction suggests that clinoform amplitude reflects the structural relief generated by tectonic wedge development and load‐induced flexural subsidence of the foreland basin.
Northern Alaska remains one of the most prospective exploration frontiers in onshore North America. In 2006, The Alaska Division of Geological & Geophysical Surveys conducted a field program in the Kavik area of the east-central North Slope, providing new constraints on the evolution of the petroleum system. This region includes two undeveloped gas discoveries (Kemik and Kavik fields) and is one of few areas that permit examination of all three depositional megasequences (Ellesmerian, Beaufortian, and Brookian) in close association. The focus of this project was to: 1) better constrain the timing and nature of regional burial and Tertiary exhumation events affecting the hydrocarbon maturation and migration history, and 2) further define the depositional environments and sequence stratigraphy of selected Brookian and Beaufortian source and reservoir units. Detailed stratigraphic and structural data, in conjunction with some of the first 1:63,360-scale geologic mapping in the area, provide insight on the complex deformation history in the region. The southern, well exposed portion of map area is dominated by detachment folding. Key structures in the Cretaceous and Tertiary strata to the north (including the trapping structure at the Kavik field) are poorly exposed, although newly acquired 2-D seismic and existing well data will allow for a more robust structural analysis. New observations of Beaufortian stratigraphy (associated with the rift-related opening of the Arctic Ocean Basin) indicate a potentially more complex paleogeography than previously assumed. Within the Cretaceous Brookian foreland succession, facies considerations suggest possible structural telescoping emplacing the informally named Juniper Creek sandstone along a detachment within the Jurassic Kingak Shale.
