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During the 2023 field season, geologists from the Alaska Division of Geological & Geophysical Surveys conducted bedrock geologic mapping in the Big Delta and Circle quadrangles in Alaska. The project areas are of interest for potential mineral resource development, including gold and copper deposits. This report provides field station locations, rock descriptions, and magnetic susceptibility measurements. The data is available for download from the DGGS website. The project aims to produce accurate geologic maps to support mineral resource exploration in eastern Interior Alaska. [Extracted from the article]

A new 1:63,360-scale geologic map of the hydrocarbon-bearing Umiat-Gubik area of the central North Slope, Alaska, spans approximately 2,100 km2 at the northern extent of the Brooks Range foothills fold-and-thrust belt in the Colville foreland basin. This geologic map was prepared through assimilation of field observations, aerial and satellite imagery, seismic-reflection data, and well logs. Near-surface formation picks were available or derived for most of the area's 24 exploration wells, and two cross sections were constructed along lines of section that are constrained at depth by our interpretations of publicly available two-dimensional seismic data. The mapped area hosts exposures of Upper Cretaceous strata in the Nanushuk, Seabee, Tuluvak, Schrader Bluff, and Prince Creek Formations, constituting an approximately 2-km-thick succession that crops out discontinuously in the low-relief, tundra-mantled region. This part of the siliciclastic Brookian megasequence stratigraphy comprises principally shallow-marine deposits. Our work benefits from and reflects recent sequencestratigraphic advances that better constrain how this part of the Colville basin continued to fill by a northeastward prograding clastic wedge during Late Cretaceous time, with the exposed stratigraphy recognized as basin-scale topset units. A series of east- to east-southeast-trending, km-scale wavelength, gentle folds are mapped in the area. Anticlines are locally breached by thrusts and interpreted to be folded above faulted and penetratively deformed mid-Cretaceous Torok Formation. Undeveloped, subcommercial (as of this writing) petroleum accumulations occur along doubly plunging anticlinal traps at three long-recognized fields in the map area: Umiat (mostly oil), Gubik (gas), and East Umiat (gas). The Umiat oil field structural culmination is modified by thrust faults that breach the surface, and the East Umiat gas field is associated with a northdipping back-thrust that is evident in seismic data and cuts across the Upper Cretaceous stratigraphy; thrust faults near the Gubik gas field lie within and below the Torok Formation. Various interpretations have previously been published for some of the area's structures, with important implications for petroleum trap geometries in the gas-prone foothills region. We present new data and interpretations that support the inference of a principal, south-dipping thrust fault that breaches the north limb of Umiat anticline near Umiat. Additionally, a new fracture dataset addresses the previously hypothesized Colville fault. Ultimately, we do not find compelling evidence for a through-going, left-lateral strike-slip fault along the Colville River valley, which extends obliquely across the structural grain of the region. The fracture data are, however, generally consistent with a pure shear model of deformation associated with north-south contraction of the central Brooks Range foothills fold-and-thrust belt. [ABSTRACT FROM AUTHOR]

The Naknek Formation records Late Jurassic forearc basin sedimentation and crops out discontinuously along an approximately 1,000 km trend in southern Alaska. Recent work in the Iniskin-Tuxedni bays area yielded depositional environment interpretations for the lower Oxfordian-lower Kimmeridgian Naknek members of Cook Inlet (in ascending order): Chisik Conglomerate (fan delta), lower sandstone (inner shelf), Snug Harbor Siltstone (outer shelf and slope), and Pomeroy Arkose (base of slope and basin floor). Geologic mapping and stratigraphic reconnaissance also led to the discovery of three seismic-scale, deep-water paleocanyons incised into Snug Harbor slope strata and chiefly filled with channelized Pomeroy sandstone. Following incisional establishment of the canyons, confinement was maintained by erosional and aggradational processes, with indications of the latter in anomalously thick successions of Snug Harbor that crop out immediately adjacent to fill strata and are interpreted as channel-belt-bounding master levees. Fill lithologies and stratigraphic architecture reflect an overall reduction of thalweg gradients following an early episode of canyon evolution marked by incision and sediment bypass. Ultimately, canyon-associated depositional systems trended toward aboveseafloor equilibrium profiles as arkosic sediment debouched onto the basin floor and canyon mouths retreated upslope. Recognition of the deep-water canyons combined with a synthesis of our depositional systems and stratigraphic architecture studies permit a sequence-stratigraphic analysis of the Naknek Formation. The basal members--Chisik Conglomerate and lower sandstone--overlie the Middle Jurassic Chinitna Formation along a sequence-bounding unconformity (basal surface of forced regression) and form a lowstand systems tract (LST-1). A transgressive surface caps LST-1 at the base of Snug Harbor (outer shelf) and is overlain by a transgressive systems tract (TST). The mid Snug Harbor (slope) exhibits a somewhat thicker-bedded and coarser-grained depositional motif, defining a highstand systems tract (HST) lying above a maximum flooding surface at the top of TST. Inception of the deep-water canyons coincided with termination of HST, forming a basal surface of forced regression that in inter-canyon areas is conformable. The lower Pomeroy and two distinct stratigraphic elements of Snug Harbor constitute base-of-slope and basin-floor components of a lowstand systems tract (LST-2). A complete third order (106 years duration) stratigraphic sequence comprises LST-1 through HST, with LST-2 occurring in the overlying sequence. Dominant sediment supply signals are recorded by the coarse, arkosic deposits of forced through normal regressions of LST-1 and LST-2, whereas TST and HST reflect episodes of potentially subtle alternation between accommodation-favored (TST) and sediment-supply-favored (HST) conditions. Establishment of a shelf-slope-basin-floor depositional profile by progradation of clinoforms during HST is inferred, with the incipient canyons cut into this HST slope. Accumulation of slope-bypassed sediment of LST-2 occurred along deep-water reaches of diminishing gradient at and beyond the base of slope, ultimately forming a coarse clastic wedge of coalesced basin-floor fan lobes that backstepped and onlapped the inherited slope profile and probably thinned basinward by downlap beyond the outcrop belt. The deep-water canyons and onlapping fan complex are stratigraphic elements and relations well known from seismic reflection datasets around the world but rarely are documented in outcrop. The Naknek Formation marked the regional onset of ubiquitous batholithic provenance in the forearc, signaling exhumation of the magmatic arc's roots and suggesting tectonism was a driving force of sediment supply during the Late Jurassic. However, there are indications that changing climate modulated Naknek sedimentation regimes, and ostensible glacioeustatic changes have been reported for Callovian-Oxfordian time that may have critically influenced base-level fall and development of the base-of-Naknek sequence boundary. Nevertheless, convergent margin tectonics likely played the dominant role in driving changes in Naknek sediment supply and accommodation, with climate and eustasy contributing subordinate influences on the resultant stratigraphic architecture. This stratigraphic analysis relates to petroleum systems of the underexplored Cook Inlet by identifying depositional dip gradient domains in the Naknek Formation, defining reaches through time and space characterized by erosion, bypass, and sediment accumulation in the context of base-level cyclicity and sediment supply. Constraints on sediment routing and timing for export of coarse detritus into marine settings improves predictability of facies distribution in the subsurface. However, Naknek sandstones contain a high proportion of labile grains, typically resulting in diagenetic destruction of porosity and permeability during burial. Nevertheless, exploration models may benefit from integrating the depositional systems work of this study, recognizing that provenance, sediment routing, lithologic parameters, and oil charge timing are keys to determining reservoir quality variability for conventional and unconventional plays. [ABSTRACT FROM AUTHOR]