Mountain Flank Research Articles

Soils Consumed by Chimpanzees of the Kanyawara Community in the Kibale Forest, Uganda

We previously reported on a study of 4 soils that chimpanzees of the Kanyawara community in the Kibale National Park, Uganda consumed on a near-daily basis. We suggested that iron was a possible chemical stimulus in association with high quantities of Si:Al = 1:1-dominated clay minerals in the consumed material. To test our initial findings, we analyzed 18 samples from the same general area including 7 samples that the chimpanzees did not eat. Among the chemical elements, As, Au, Br, Ca, Cl, Dy, Mg, Ni, Sb, Sr, and I are below detection limits. Only Fe stands out as a potentially important nutritional element present in sufficient quantity to provide a physiological stimulus for chimpanzees living at high elevations near the flanks of the Ruwenzori Mountains. Along with Fe, metahalloysite is present in high amounts in these soils. In its pure crystalline form as a pharmaceutical grade clay mineral metahalloysite may well counteract the debilitating effects of diarrhea, with an effect similar to what is achieved with kaolinite (cf. Kaopectate TM ). An unexpected result, the relatively high nitrogen and carbon in the eaten samples relative to the uneaten group, indicates the chimpanzees may have a higher threshold for organic-rich material than previously believed. Contrarily, the color of the ingested material, depicts a material with less humus than in the uneaten group, a finding that is compatible with previous work reported at other geophagy sites in Africa. Of all the choices of soil available to them, the chimpanzees appear to be selecting highly homogeneous chemical natural earths with well-leached and uniform mineralogical material similar to the uneaten group, but with higher relative amounts of clay size material.

The subgrid‐scale orographic blocking parametrization of the GEM Model

The impact of the physical parametrization called subgrid‐scale orographic blocking, recently introduced in the physics of the Canadian Global Environmental Mutiscale (GEM) model is described. It is based on a formulation by Lott and Miller (1997) and represents the unresolved component of the drag on low‐level winds that are blocked at the flanks of mountains. The blocking term plus the gravity‐wave drag are now part of a unified parametrization of the subgrid orographic drag that became operational in the global GEM model on 11 December 2001. Results from tests made with various configurations of the model are shown, illustrating how the blocking term impacts the large‐scale flow and improves both the short‐ and the medium‐range forecasts, especially in winter. It is shown that at day 5 of the model integrations, the influence of the blocking force applied near the surface is felt by the entire tropospheric and the lower‐stratospheric circulation. A mechanism based on perturbations of the Eliassen‐Palm flux caused by the low‐level forcing is proposed to explain the vertical propagation of the signal generated by the blocking term.

Talus fabric, clast morphology, and botanical Indicators of Slope Processes on the Chaos Crags (California Cascades), U.S.A.

The Chaos Crags, a group of dacite domes in the Cascades Mtns (California), were affected by volcanic debris avalanches ca. 1675 A.D.; these left a sizable deposit and a scar on the north mountain flank, now covered by talus. This report examines the fabric and morphology of talus debris, their spatial variation, and the geomorphic processes presently affecting the slope. The talus presents a bi-segmented profile with a steep upper rectilinear segment and a shorter concave, basal zone. Debris are sorted by size both along (larger clasts downslope) and across the talus (larger particles below the cliffs). Shape sorting is weaker, but clast sphericity increases, and elongation decreases, toward the footslope. Upper-talus fabrics (long axes parallel to talus plane and slope) show that clasts there move by sliding, while basal blocks are deposited by rockfall, which causes more iso- tropic fabrics. Field observations and botanical evidence indicate the overall significance of grain flows, which are prevalent duetoanabundantsupplyofrubbleonthe upper talus. Comparison with similar recent slopes and repeat photography suggest the Chaos talus formed swiftly following dome collapse, when much debris may have collected below the unstable rockwalls. The talus has experienced only modest sedimentation during the 20 th century, and is currently affected by 'normal' mass-wasting processes, which also include snow avalanches and debris flows.

Tectonically driven landscape development within the eastern Alpujarran Corridor, Betic Cordillera, SE Spain (Almerı́a)

The Betic Cordillera is a topographic manifestation of Miocene to recent collision between the African and the European plates. Mountain ranges in the Internal Zone of the Betic Cordillera are exhumed massifs of metamorphic and sedimentary rocks, and are surrounded by low-lying sedimentary basins. The Neogene history of mountain range uplift and basin formation in the Betics is well studied, but Quaternary topographic development and its relation to convergent plate boundary tectonics is less understood. Deformation, rates of stream incision and factors controlling landscape evolution on the flanks of the highest mountains in the Betic Cordillera are studied to gain insight on the relationship of Quaternary tectonics to topographic development. Specifically, geomorphic processes that led to formation of topographic relief within the Internal Zone of the Betic Cordillera are assessed. This paper is based on a field study conducted in the eastern Alpujarran Corridor (EAC), which is the eastern half of a 4–6-km wide and 80-km long inverted sedimentary basin. The Mid to Late Quaternary history of basin denudation is deduced on the basis of morphostratigraphic, geochronologic and structural–geologic analyses of oxygen isotope Stage 8-time fluvial deposits in the EAC. Results of these analyses indicate that climate and faulting played only a minor role in landscape evolution in the EAC. Instead, the EAC topographic trough was excavated by a drainage network whose axial stream (Rı́o Andarax) was incising at a rate between 0.3 and 0.7 m/ka since oxygen isotope Stage 8. This paper demonstrates that these incision rates are the same as regional rock uplift rates in the EAC. During the Mid and Late Quaternary, topographic relief between the EAC and adjacent mountain ranges developed largely as a result of fluvial denudation driven by regional uplift. The implications of the results of this investigation are used to propose a model of Quaternary topographic relief development in the Internal Zone of Betic Cordillera. Topographic relief between Internal Zone bedrock massifs and adjacent basins formed largely as a result of fluvial denudation driven by regional uplift. Faulting also played a significant role in areas that are within a few kilometers of crustal-scale fault zones.

Geomorphic indicators of active fold growth: South Mountain–Oak Ridge anticline, Ventura basin, southern California

South Mountain–Oak Ridge, near Ventura, California, is an asymmetric anticlinal uplift forming at the present time above the active, buried Oak Ridge reverse fault. Shortening along the Oak Ridge fault accumulated largely in Quaternary time and is responsible for the growth and present topography of the westernmost 15 km of the ridge during the past 0.5 m.y. Tectonic geomorphic analysis using several indices of active tectonics provides information concerning fold growth. Stream-gradient indices are relatively high in the northern, eroded fold scarp of the ridge, a pattern consistent with the existence of active, rapid slip on the Oak Ridge fault. Mountain-front sinuosity along the northern slope of the anticlinal ridge roughly decreases from ∼2 to 1 toward the westernmost 10 km of observed surface folding. Valley floor width to valley height ratios along the northern flank of the ridge generally decrease westward from ∼1.5 to 0.5. Values of the hypsometric integral along the northern flank increase significantly from ∼0.35 to 0.4 (maximum ∼0.55) from east to west. Drainage density varies from ∼4 to 6 km/km2 along both flanks of the South Mountain–Oak Ridge anticline. Entrenchment of streams into the (southern) backlimb of the fold along the westernmost 9 km of the structure decreases from ∼20 m to <1 m from east to west. Apparent backlimb rotation, as measured by dip of strata along the westernmost 7 km of the fold, decreases from east to west, from ∼35° to 20°. Fold growth of the South Mountain–Oak Ridge anticline occurred during the past 0.5 Ma following deposition of the Saugus Formation. Lateral and vertical fold growth were likely produced by westward decrease in fault slip along the buried Oak Ridge fault.

The state of stress in the limb of the Split Mountain anticline, Utah: constraints placed by transected joints

Transected joints (i.e. systematic joints that strike at an angle to the present fold axis trend) occur on the flanks of Split Mountain, a Laramide anticline near the eastern end of the Uinta Mountains, Utah. The common orientation on both flanks for these WNW-striking joints is inconsistent with joints driven by a syn-folding stretch normal to the direction of highest curvature. A smaller dispersion of the poles to these transected joints occurs when they are rotated with bedding to their ‘pre-fold’ orientation. This dispersion of poles is inconsistent with a post-fold genesis in a regional stress field but permits the possibility that these WNW joints propagated as a systematic set prior to Laramide folding. A pre-fold interpretation is substantiated by a regional WNW-striking joint set within Cretaceous and older rocks in the surrounding Piceance, Uinta, and southeastern Sand Wash basins. During tilting accompanying the upfolding of Split Mountain, most joints of this WNW-striking regional set remain locked without slipping under a shear stress. Fracture toughness and frictional strength are two rock properties that serve to lock a joint until a critical resolved shear stress is achieved. A gravity load caused down-dip slip on some joints that were tilted to a dip of about 62°. This suggests that a local principal stress remained roughly vertical during bedding rotation. Assuming fracture strength and friction prevented slip on most joints during tilting, the ratio of least horizontal, Sh, to vertical stress, Sv, at the critical tilt angle was approximately 0.55.

Book Review

Book Review

Role of extreme meteorological anomalies in initiating the Darbang Landslide, Dhaulagiri Himal, Western Nepal

A catastrophic landslide occurred at midnight of 20 September 1988 at a small village of Darbang (1130 m above mean sea level) in western Nepal. The village is situated approximately 40 km northwest of the district capital Baglung. Farmhouses spread out on both sides of the Myagdi Khola, a main tributary of the Kali Gandalki River.&#x0D; An overly steep and more than 800 m high northern flank of a mountain collapsed spontaneously. Approximately 5 million m3 of the displaced material blocked the valley of the Myagdi Khola and dammed the river for 6 hours. The eventual breakup of the landslide dam caused extreme flooding in the lower reaches. Many houses of Darbang, particularly on the right bank of the Myagdi Khola, were destroyed. More than 100 persons lost their lives. This catastrophic event could be interpreted as a revisit of the devastating landslide of 1926, which killed more than 500 villagers.&#x0D; Diverse lithology (phyllites, schists, and quartzites), presence of the MCT in the vicinity, extremely high relief; and environmental degradation (e.g., deforestation), are regarded as the primary causative factors of the Darbang Landslide.&#x0D; Yet the decisive factor behind the massive landslide of 1988 was the unique meteorological situation of that year. The meteorological data show that, during the pre-monsoon months of April, May, and June 1988, the maximum temperature was 2° C higher than the average for the decade. Monthly precipitation measured in May and June of 1988 showed the totals respectively 37% and 24% below the decade-average. It led to extreme aridity in the period preceding the onset of the seasonal monsoons. Soils dried out and deep cracks were formed in this process. During the heavy summer rainfall of 1988 the cracks functioned as conduits of water. The bedrock contained numerous fractures and joints, through which the .vater seeped deep into it. Extremely heavy precipitation in August, with rainfall of up to 17% above the decade-average, aggravated this delicate situation. In the first half of September 1988 precipitation peaked. These meteorological conditions, coupled with the already saturated bedrock, and deteriorated natural vegetation lead to the collapse of the mountain flank.&#x0D; Primary causative factors and environmental degradation over a longer period were underlying reasons for the initial weakening of the site, but the unique weather situation in 1988 was the main trigger of this catastrophic event.

Chronology of Neogene and Quaternary uplift and magmatism in the Caucasus: constraints from K–Ar dating of volcanism in Armenia

The Greater Caucasus is one of Earth's highest actively-uplifting mountain ranges; the adjoining Caspian Sea basin contains a substantial proportion of its hydrocarbon reserves. Like other parts of the former Soviet Union, the Neogene and Quaternary chronology of these important regions has not previously been well-defined. It has thus been impossible to obtain reliable estimates for rates of processes such as uplift of the Caucasus and sedimentation in the Caspian Sea. Previous studies have established the relative timings of events in the region, using correlation schemes between volcanism, glaciations, and the stratigraphy of the Caspian basin. However, a range of absolute chronologies has previously been proposed for these sediments and igneous rocks, based mainly on different interpretations of their magnetostratigraphic records. By K–Ar dating, we determine the ages of volcanism at three localities in Armenia as 1.1, 0.8 and 0.8 Ma. Using these data and other evidence, we propose a revision to the chronology of this region, in which a distinctive brief interval of normal magnetic polarity in the local sedimentary and volcanic magnetostratigraphic records is matched to the Cobb Mountain event in the global record rather than the Olduvai event or an earlier subchron as had previously been thought. We thus interpret a ∼1.5 Ma timing for the start of volcanism in the Lesser Caucasus, and also suggest a ∼1.2 Ma timing for the Late Akchagyl transgression of the Caspian Sea, a key event in the regional stratigraphy when this water body reached its greatest extent. We tentatively correlate this transgression with the melting event following glaciation during stage 36 of the oxygen isotope timescale, which was thus the first time during the Pleistocene when eastern Europe was covered by a lowland ice sheet. Time-averaged since ∼1 Ma, the flanks of the eastern Greater Caucasus mountains are shown to have uplifted at ∼0.6 mm a −1 and the Lesser Caucasus at ∼0.3 mm a −1. We show that the rate and spatial scale of this uplift are too great to be the result of plate convergence, and suggest instead that this uplift is caused by crustal thickening due to inward lower-crustal flow to beneath these mountain ranges. At the start of magmatism in both the Greater and Lesser Caucasus, the estimated crustal thickness was ∼45 km. We thus suggest that this magmatism has been caused by heating of the mantle lithosphere due to earlier crustal thickening, the temperature rise required to initiate magmatism being the same in both cases.

Development of potential acid sulfate paleosols in Paleocene floodplains, Bighorn Basin, Wyoming, USA

Floodplain paleosols in the Paleocene Fort Union Formation in the Bighorn Basin are distinguished by a dark claystone interval with yellow jarosite mottles, relatively high total organic carbon values, and woody debris and leaves along bedding planes. The paleosols also have various redoximorphic features, including low matrix chromas, grey root mottles, and local concentrations of goethite and hematite, especially associated with roots and other organic matter. Jarosite is usually an oxidation product of pyrite, which forms under waterlogged conditions when sulfate and organic matter are available. The former presence of pyrite and the redoximorphic features suggest that the Fort Union paleosols first developed as potential acid sulfate soils ( Brinkman and Pons, 1973). The dark claystones with jarosite are interpreted as permanently reduced or Gr horizons. The Gr horizon is overlain by mudstone that is lighter colored and lacks jarosite. This represents an ancient Bg horizon or A/Bg sequence. The jarosite indicates that the paleosols underwent a subsequent episode of oxidation and acidification. Those processes require that the soils be drained, which probably did not occur during Paleocene time and may be a recent event. Rather, the soils were probably buried as the Fort Union Formation continued to aggrade, and the waterlogged soils were pushed even farther below the groundwater table. Only with late Cenozoic excavation of strata in the basin and exposure of the Fort Union deposits, did jarosite precipitate. Pedogenic and paleobotanical features suggest that the soils formed in floodplain swamps dominated by coniferous trees. The high organic content and clay-rich nature of the Gr horizons suggest that the floodplain swamps were not close to the active channel. The bipartite nature to the profiles (Bg horizon or A/Bg sequence above a Gr horizon) probably resulted from two different kinds of floodplain deposition. Grain size trends indicate that the Gr horizons formed on sediment deposited by overbank flooding. The impermeable, fine-grained sediment and its low topographic position led to waterlogging. The overlying A/Bg or Bg horizons formed on coarser sediment that was probably deposited during channel avulsion. The greater permeability and higher topographic position of this sediment caused it to be better drained than the Gr horizon. The paleosols are unusual because they formed in a freshwater setting, whereas modern potential acid sulfate or acid sulfate soils form in coastal areas where sulfate is readily available. The source of the sulfate may have been Cretaceous marine shales, exposed along the flanks of mountains rising around the Bighorn Basin during Paleocene time. Other Upper Cretaceous and Paleocene formations in the Rocky Mountain area contain similar rocks, suggesting that sulfidic soils were relatively common in the mid-continent at this time. Most of these other examples probably reflect deposition associated with the retreating Western Interior Seaway and humid climates.

The Mountain Flank - Fall and Winter

The Mountain Flank - Fall and Winter

Late Holocene, high-resolution glacial chronologies and climate, Kenai Mountains, Alaska

Recent retreat of outlet glaciers from the Harding and Grewingk-Yalik Icefields has revealed a vast array of deposits on the eastern and western flanks of the Kenai Mountains that records multiple glacier advances into coastal forests during late Holocene time. Treering dating, together with radiocarbon and lichenometric analyses, allows for the reconstruction of these glacial fluctuations to decadal precision over the past two thousand years. The records of fluctuations are derived from 16 land-terminating and seven tidewater glaciers in three fjord systems, as well as two cirque glaciers. Three major intervals of Holocene glacier expansions are evident; they occurred about 3600 yr B.P., 600 A.D., and during the Little Ice Age, from 1300 to 1850 A.D. The earliest expansion beyond present ice margins is known only from the McCarty tidewater glacier. The 600 A.D. event involved the simultaneous advance of land-terminating and tidewater glaciers. During the Little Ice Age, however, tidewater glaciers were advancing several centuries prior to their land-terminating neighbors. Those land-terminating glaciers on the western mountain flank retreated from their Little Ice Age maxima as much as two centuries before those on the eastern mountain flank. Land-terminating tongues on the eastern, more maritime, mountain flank have shown more sensitivity to variations in winter precipitation during the Little Ice Age and within recent decades than the more continental glaciers on the western flank that are affected more by summer temperatures. The glacial and climatic records suggest that advances of the ice tongues from about 1420 to 1460 A.D., between 1640 and 1670 A.D., at about 1750 A.D., and from 1880 to 1910 A.D. reflected times of increased winter precipitation. Advances between 1440 to 1460 A.D., from 1650 to 1710 A.D., and from 1830 to 1860 A.D. followed intervals of lower summer temperatures.

Structural Style of East Flank of Bighorn Mountains, Johnson and Sheridan Counties, Wyoming: ABSTRACT

The 70 mi-long portion of the east flank of the Bighorn Mountains, between Sheridan and Mayoworth, Wyoming, is structurally divisible into three distinct segments - northern, central, and southern - each distinguished by a dominant sense of vergence and structural style. The northern segment displays southwest-verging reverse faults and associated folds, indicating tectonic transport out of the Powder River basin and onto the mountain flank. The central segment displays northeast and east-northeast-verging reverse faults and associated folds, indicating tectonic transport of the mountain flank over the Powder River basin. Seismic and drill-hole data indicate most of these reverse faults dip to the southwest and west-southwest at angles of 35{degree} or less. The southern segment displays west-southwest-verging reverse faults and associated folds, again indicating tectonic transport out of the Powder River basin and onto the mountain flank. All major structures identified within the area of investigation are basement involved, and the geometry of the rocks supports the concept that the mountain flank deformed under the influence of northeast-southwest-directed horizontal compression rather than vertically oriented block uplift.

Geochemical studies on Kashmir loess profiles

Major and trace element concentrations of a few representative loess profiles in Kashmir valley are studied using EDXRF spectroscopy. Principal component analysis for pattern recognition has been employed to study association between elemental concentration variations in various horizons. The present study reveals that elements related to clay illuviation (K, Rb, Fe, Ti, Mn) and plant activity (Cu, Zn) are generally higher in B horizon of the palaeosols whereas Ca and Sr have a high concentration in the loess layers. Such an elemental concentration variation provides us with an objective index for identification and confirmation of palaeosols. There is further a close association in the movement of (i) K, Rb, (ii) Fe, Ti, Mn, (iii) Cu, Zn and (iv) Ca, Sr. These clusters of elements are also present in modern soils, suggesting that the soil characteristics in palaeosols have been preserved. Concentration of iron and titanium is also observed to be higher in loess profiles on Pir Panjal mountain flank which is dominated by basic rocks, compared to profiles on Himalayan flank, suggesting that the loess in Kashmir might have been derived from the nearby rocks in a localized way.

Oral Tradition, Genealogy and Korean Village Morphology

Korean rural settlements are generally clustered at midslope sites on the flanks of hills or mountains. The settlement sites and forms were determined both by Fengshui and by palrilineal inheritanc...

Balanced Cross Sections of Small Fold-thrust Structures

Balanced, restorable cross sections for the Precambrian-cored Sage Creek anticline in the Wind River Basin are combined with constraints on the mechanical behavior of various types of basement rocks and sedimentary rocks to give a new model of the apparent folding of the upper basement surface and the early stages of a fold-thrust structure. Vertical relief on the upper basement surface develops as a result of motion along a zone of discrete, parallel faults, giving an upper basement surface which appears to be folded on some scale of observation. The sedimentary rocks above the uplift are folded into a tight anticline-syncline pair. Progressive restoration of the cross sections using the proposed geometry of the early stages of uplift shows that folding and overturning above the fault zone is the major component of shortening in the cover rocks. The late "mountain flank" thrusts, which cut the fold increase the vertical relief of the upper basement surface and extend the cover rocks which increases the fold amplitude in the cover rocks. Cross sections depicting the present geometry must balance throughout the development of the structure and satisfy the predicted mechanical behavior of the rocks involved at the conditions under which the structure formed.

Accumulation of organic matter in Cretaceous oxygen-deficient depositional environments in the central Pacific Ocean

Complete records of organic-carbon-rich Cretaceous strata were continuouslycored on the flanks of the Mid-Pacific Mountains and southern Hess Rise in the central North Pacific Ocean during DSDP Leg 62. Organic-carbon-rich laminated silicified limestones were deposited in the western Mid-Pacific Mountains during the early Aptian, a time when that region was south of the equator and considerably shallower than at present. Organic-carbon-rich, laminated limestone on southern Hess Rise overlies volcanic basement and includes 136 m of stratigraphic section of late Albian to early Cenomanian age. This limestone unit was deposited rapidly as Hess Rise was passing under the equatorial high-productivity zone and was subsiding from shallow to intermediate depths. The association of volcanogenic components with organic-carbon-rich strata on Hess Rise in the Mid-Pacific Mountains is striking and suggests that there was a coincidence of mid-plate volcanic activity and the production and accumulation of organic matter at intermediate water depths in the tropical Pacific Ocean during the middle Cretaceous. Pyrolysis assays and analyses of extractable hydrocarbons indicate that the organic matter in the limestone on Hess Rise is composed mainly of lipid-rich kerogen derived from aquatic marine organisms and bacteria. Limestones from the Mid-Pacific Mountains generally contain low ratios of pyrolytic hydrocarbons to organic carbon and low hydrogen indices, suggesting that the organic matter may contain a significant proportion of land-derived material, possibly derived from numerous volcanic islands that must have existed before the area subsided. The organic carbon in all samples analyzed is isotopically light ( δ 13C − 24 to − 29 per mil) relative to most modern rine organic carbon, and the lightest carbon is also the most lipid-rich. There is a positive linear correlation between sulfur and organic carbon in samples from Hess Rise and from the Mid-Pacific Mountains. The slopes and intercepts of C-S regression lines however, are different for each site and all are different from regression lines for samples from modern anoxic marine sediments and from Black Sea cores. The organic-carbon-rich limestones on Hess Rise, the Mid-Pacific Mountains, and other plateaus and seamounts in the Pacific Ocean are not synchronous but do occur within the same general middle Cretaceous time period as organic-carbon-rich lithofacies elsewhere in the world ocean, particularly in the Atlantic Ocean. Strata of equivalent age in the deep basins of the Pacific Ocean are not rich in organic carbon, and were deposited in oxygenated environments. This observation, together with the evidence that the plateau sites were considerably shallower and closse to the equator during the middle Creataceous suggests that local tectonic and hydrographic conditions may have resulted in high surface-water productivity and the preservation of organic matter in an oxygen-deficient environment where an expanded mid-water oxygen minimum developed and impinged on elevated platforms and seamounts.

Late Tertiary history of the Antarctic ice sheet: Evidence from the Dry Valleys

Data from the Dry Valleys, Antarctica, suggest that outlet glaciers of a local ice cap carved primary valley systems on both flanks of the Transantarctic Mountains, while coeval alpine glaciers eroded exposed intervalley mountain ranges. Subsequently, a thick ice sheet overrode the Transantarctic Mountains at least twice, flowing northeastward across major pre-existing valleys. The youngest overriding episode postdated and the older episode antedated middle to early late Miocene time. We think that an extensive ice sheet covered East and West Antarctica during overriding episodes.

Basement Fault Configurations, Wyoming Province: ABSTRACT

For many basement-cored folds and related mountain flank faults in the Wyoming province, locally balanced cross sections can be constructed using structural relief and line lengths in the sedimentary cover rocks. By conserving the length of the upper basement surface, possible basement fault movement can be inferred. This faulting includes: (1) motion along a reverse fault which yields a folded upper basement surface, and (2) displacement distributed along a series of parallel basement faults. The sedimentary cover rocks are force folded and may be cut by mountain flank faults which place Precambrian rocks overlying either Paleozoic or Mesozoic rocks. Previously recognized thrusts, blind thrusts, rootless anticlines, and buckle folds in the cover rocks in adjacent basins re reinterpreted to represent thin-skinned deformation related to basement faulting. Where there is relative translation of the cover rocks, cross sections need not balance locally and the fault dips and basement geometries previously determined are in error. The fault dips calculated from locally balanced cross sections have been overestimated, and/or the displacement on the basement fault(s) has been underestimated. The displacement observed on mountain flank faults may be far less than the total displacement on the basement fault. In the simplest case, it may be possible to project the mountain flank fault downdip to infer the orientation and location of the basement fault(s) at depth. End_of_Article - Last_Page 950------------

Oil and Gas Prospecting Beneath Precambrian of Foreland Thrust Plates in Rocky Mountains

Only 16 wells in the Rocky Mountain region have drilled through Precambrian rocks to test the 3 to 6 million acres of sedimentary rocks that are concealed and virtually unexplored beneath mountain-front thrusts. One recent test is a major gas discovery, another a development oil well, and over half of the unsuccessful tests had oil or gas shows. These wells have not only set up an exciting play, they have also helped define the structural geometry of the mountain-front thrusts, including the angle of the thrust, the amount of horizontal displacement, and the presence or absence of fault slivers containing overturned Mesozoic or Paleozoic rocks. Important for further geophysical exploration, these wells have provided vital data on seismic velocities in Precambrian rocks. A alysis of these data has stimulated further exploration along the fronts already drilled: in Wyoming, the Emigrant Trail thrust, the Washakie thrust, the Wind River thrust, the thrust at the north end of the Laramie Range, and the Casper arch; in Utah and Colorado, the Uncompahgre and Uinta uplifts. The geologic success of these wells has encouraged leasing and seismic acquisition on every other mountain-front thrust in the Rockies. An unsuccessful attempt to drill through the Arlington thrust of the Medicine Bow Range will probably only momentarily daunt that play, and the attempted penetration of the Axial arch in Colorado has not condemned that area; in fact, another well is being drilled at this time. Untested areas that will be explored in the near future are: in Wyoming, the south flank of the Owl Creek Range, the southwest flank of the Gros Ventre Range, the east and west flanks of the Big Horn Mountains, the west flank of the Big Horn basin, the north flank of the Hanna basin; in Utah, the south flank of the Uinta Mountains; and in Colorado, the White River uplift, the no th flank of North Park basin, and the Front Range.