Blackhawk Mountain in southern California rises above southeastern Lucerne Valley at the eastern end of the rugged 4,000-foot escarpment that separates the San Bernardino Mountains on the south from the Mojave Desert on the north. Its summit is a resistant block of marble thrust northward over easily eroded uncemented sandstone and weathered gneiss. Spread out on the alluvial apron at the foot of the mountain is the prehistoric Blackhawk landslide, a lobe of nearly monolithologic marble breccia from 30 to 100 feet thick, 2 miles wide, and 5 miles long. The Blackhawk landslide and an adjacent older landslide, the Silver Reef, have many peculiarities of form and structure in common with the historic Elm, Frank, and Sherman landslides; and in lithology, provenance, size, and “coefficient of friction” they strongly resemble many of the monolithologic breccia deposits of possible landslide origin found associated with Tertiary faults and fanglomerates in the southwestern United States and elsewhere. The older rocks of the Blackhawk area consist of gneiss, quartzite, Carboniferous marble, and Cretaceous quartz monzonite, which originally underlay a landscape of relatively low relief. Uplift of Blackhawk Mountain, first by overthrusting from the south, and then by monoclinal folding along a northwest-trending axis, led in the late Tertiary and Quaternary to deep erosion of the mountain front accompanied by the growth northward of extensive alluvial deposits interspersed with several large-scale landslides, the largest and most recent of which is the Blackhawk landslide. Both the geological evidence and, in the case of the Elm and Frank landslides, the eyewitness reports suggest that the Blackhawk landslide and its congeners started as huge rockfalls, which were launched into the air and then traversed the gently inclined, relatively smooth slopes below as nearly nondeforming sheets of breccia sliding at high speed on a relatively thin, easily sheared lubricating layer. These facts suggest the hypothesis that landslides of this type acquire such high speed in their descent that at a sudden steepening of slope they leave the ground, overriding and compressing a cushion of trapped air upon which they traverse the gentler slopes below with little friction, much as the slipper in a thrust bearing slides on a cushion of oil with no metal-to-metal contact. This air-layer lubrication readily accounts for the low friction, high speed, and nonflowing motion of these large landslides and explains many otherwise puzzling details of their form and structure, such as the striking three-dimensional jigsaw puzzle effect seen in the pervasively fractured larger blocks, the transverse corrugations, soil schlieren, and certain of the peculiar debris cones on the landslide surface, the moraine-like ridges along the sides, and the low rim, steep scarp, and transported debris at the distal end. Thus, it appears that under the right circumstances massive avalanches like the Blackhawk can slide for miles on nothing more substantial than an ephemeral layer of compressed air.

Abstract The Upper Devonian and Lower Mississippian Bakken Formation in the United States part of the Williston Basin is a giant continuous (unconventional) oil resource. A recent U.S. Geological Survey (USGS) assessment estimated a mean volume of undiscovered technically recoverable oil for the Bakken Formation of about 3.65 billion bbl of oil. The estimate is based on a geologic model and a methodology that defines different assessment units by accumulation type (conventional or continuous), structural control, fracture occurrence and prediction, lithology and petrophysical properties, formation thickness, underlying salt movement or dissolution, and level of thermal maturity and oil-generation capacity of Bakken source rocks. The Bakken Formation consists of three informal members: (1) lower shale member; (2) middle sandstone member; and (3) upper shale member. Shale members are rich in marine organic matter (as much as 35% by weight) and are the petroleum source rocks, whereas the middle sandstone member varies in depositional facies and lithology and locally exhibits good matrix porosity (as much as 14%) but with low permeability, a characteristic of tight reservoirs. Additional commingled production occurs locally from matrix porosity in the immediately underlying, informally named, Sanish sand unit of the Upper Devonian Three Forks Formation. Combined, the Bakken Formation and Sanish sand define the Bakken composite continuous reservoir. On a larger scale, thermally mature organic-rich Bakken shale members are also the source for oils produced from locally occurring Waulsortian mounds or porous strata immediately above the upper shale member in the overlying Lower Mississippian Lodgepole Limestone. As a whole, elements of petroleum source, reservoir, seal, migration, and trap define the stratigraphic and geographic character of a Bakken-Lodgepole Total Petroleum System. The geographic extent of the continuous oil accumulation within the United States part of the Bakken Formation is defined as the area in which organic-rich shale members of the Bakken Formation are thermally mature with respect to oil-generation. The area of the oil-generation window for the Bakken Formation continuous reservoir was determined using a combination of the following: (1) contour mapping of both the hydrogen index (HI) and log-resistivity well data of the upper shale member, (2) calibration of HI to the transformation ratio (TR) from one-dimensional burial history models, and (3) calibration of HI to total organic content. The geologic model used to further define continuous assessment units (AUs) within the Bakken Formation continuous oil accumulation was, in general, based on assumed levels of thermal maturity and generation capacity of the Bakken shale members as determined from HI and TR, relation of HI and TR to potential fracturing and structural complexity of the Williston Basin, and lithofacies distribution and petrophysical character of the middle sandstone member. The area of the oil generation window was divided into five continuous AUs: (1) Elm Coulee-Billings Nose AU, (2) Central Basin-Poplar Dome AU, (3) Nesson-Little Knife Structural AU, (4) Eastern Expulsion Threshold AU, and (5) Northwest Expulsion Threshold AU. One hypothetical conventional AU, a Middle Sandstone Member AU, was defined external to the area of oil generation. Using the established U.S. Geological Survey methodology, assessment of each Bakken continuous AU was performed after estimation of effective well drainage areas, estimated ultimate recovery (EUR) from productive wells, and production success defined by a minimum EUR of 2000 bbl of oil. The AUs with the greatest resource potential are the Eastern Expulsion Threshold AU (mean volume, 0.973 billion bbl of oil), which is best represented by the Parshall and Sanish fields of Mountrail County, North Dakota, and the Nesson-Little Knife Structural AU (mean volume, 0.908 billion bbl of oil), where structural reservoir development exists, the middle sandstone member is thick and porous, the underlying Sanish sand reservoir unit is commonly present, and shale members have high oil-generation potential and the probability of abundant natural fracturing.

ABSTRACT On a regional geological map, western Wisconsin looks as if it has very simple, even boring, geology. It is dominated by flat-lying, layer-cake Ordovician sedimentary rocks thinly overlain by Pleistocene glacial drift, yet detailed investigation reveals many interesting geologic features, furnishing research projects and teaching examples for all levels of geological education. In this “simple” area around Spring Valley, Wisconsin, you will see a major cave, an old mine, a large earthen dam and a meteorite impact site. In addition to what these sites say about the area’s geologic and human history, they furnish insight into how geologists piece together evidence as well as illustrate relevant subjects such as groundwater supply, control of catastrophic flooding and intelligent land use. It is our hope that, while enjoying learning about the features in this scenic part of Wisconsin, many of you will be inspired to seek out the unique geology of your area, and bring that knowledge into your classrooms.