Showing papers in "AAPG Bulletin in 1966"

Diagenesis of Gulf Coast Clayey Sediments and Its Possible Relation to Petroleum Migration

Abstract: The writer has established reference intervals in the subsurface on the basis of apparent systematic interlayer water loss of swelling clay minerals. The intervals are used in much the same manner as the familiar indicators for metamorphism, but are present at sufficiently shallow depths to be evident within oil-bearing strata of the Gulf Coast. The resulting conclusion is that clay-mineral diagenesis indicators may prove to be important petroleum-evaluation markers as well as fundamental properties of sedimentary basins. Sedimentary basins are viewed as combinations of gases, liquids, and semisolids distributed through a solid matrix. During geologic development the interstitial components segregate by migration and produce various commercially exploitable concentrations. Water, the principal fluid component of the sedimentary section, is thought to migrate in three separate stages. Initially, pore water and excessive (more than two) clay-water interlayers are removed by the action of overburden pressure. This initial water flow (which is essentially completed after the first few thousand feet of burial) reduces the water content of the sediment to about 30 percent, most of which is in the semi-solid interlayer form. A second stage of dehydration is thought to occur when the heat absorbed by the burie sediment becomes sufficiently great to mobilize the next-to-last water interlayer in an M(H2O)x + ^DgrHr = 1 + XH2O fashion. The final stage of sediment dehydration which extracts the last remaining water monolayer from clay lattices is apparently very slow, even by geologic standards, requiring tens or possibly hundreds of millions of years depending upon the geothermal and burial history of the sediment. Petroleum hydrocarbons which are distributed throughout the matrices of potential source beds in normal frequencies of 300-3,000 ppm are thought to be too sparse to initiate continuous fluid flow. In normal marine sediments, however, the water associated with clay minerals is present to a considerable depth in the order of 200,000 ppm, and therefore it is reasoned that this phase forms the connection between petroleum source and reservoir beds. The first and last dehydration stages are probably unimportant in Gulf Coast oil migration, inasmuch as they occur, respectively, at levels too shallow and too deep to intersect the interval of maximum liquid petroleum availability. The amount of water in movement during the second stage, at a level which does intersect this interval, is 10-15 percent of the compacted bulk volume and represents a significant fluid displacement capable of redistributing any mobile subsurface component. A measure of the degree to which the second-stage interlayer water has been discharged into the system can be noted on X-ray diffractograms. The movement appears to occur in a relatively restricted, depth-dependent temperature zone in which the average dehydration temperature of the points measured is 22 °F. With the use of an empirically derived P/T curve and a geothermal-gradient map, a set of regional subsurface dehydration contours can be constructed. A plot of 5,368 liquid petroleum production depths referenced to this dehydration "surface" shows an almost perfect Gaussian distribution. It seems significant that, although the dehydration depths range from 4,000 to 16,000 ft, hydrocarbon production depths are distributed in a statistically consistent relation to the calculated clay-dehydration contour surface.

Theoretical Considerations of Sealing and Non-Sealing Faults

Abstract: Differentiating between sealing and non-sealing faults and their effects on the subsurface is a major problem in petroleum exploration, development, and production. The fault-seal problem has been investigated from a theoretical viewpoint in order to provide a basis for a better understanding of sealing and non-sealing faults. Some general theories of hydrocarbon entrapment are reviewed and related directly to hypothetical cases of faults as barriers to hydrocarbon migration and faults as paths for hydrocarbon migration. The phenomenon of fault entrapment reduces to a relation between (1) capillary pressure and (2) the displacement pressure of the reservoir rock and the boundary rock material along the fault. Capillary pressure is the differential pressure between the hyd ocarbons and the water at any level in the reservoir; displacement pressure is the pressure required to force hydrocarbons into the largest interconnected pores of a preferentially water-wet rock. Thus the sealing or non-sealing aspect of a fault can be characterized by pressure differentials and by rock capillary properties. Theoretical studies show that the fault seal in preferentially water-wet rock is related to the displacement pressure of the media in contact at the fault. Media of similar displacement pressure will result in a non-sealing fault to hydrocarbon migration. Media of different displacement pressure will result in a sealing fault, provided the capillary pressure is less than the boundary displacement pressure. The trapping capacity of a boundary, in terms of the thickness of hydrocarbon column, is related to the magnitude of the difference in displacement pressures of the reservoir and boundary rock. If the thickness of the hydrocarbon column exceeds the boundary trapping capacity, the excess hydrocarbons will be displaced into the boundary material. Dependent on the conditions, lateral m gration across faults or vertical migration along faults will occur when the boundary trapping capacity is exceeded. Application of the theoretical concepts to subsurface studies should prove useful in understanding and in evaluating subsurface fault seals.

Sedimentology and Dispersal pattern of a Cretaceous Flysch Sequence, Patagonian Andes, Southern Chile

Abstract: The Upper Cretactous Cerro Toro Formation is a flysch-like sequence overlain transitionally by molasse lithologic types and including conformable detached masses of coarse detritus emplaced in part as submarine mudflows. Sediment was dispersed in a pattern that reflects both a general eastward slope, indicated by apparent gravity-controlled structures, and the action of the dominant south-flowing currents. Axial trends and overturn directions of synsedimentary folds, trends of slide channels, overturn directions of flame structures, some clast imbrication, and one example of giant flutings on conglomerate bed soles suggest west-to-east movement of material. However, most current structures, including flute casts, current-ripple and convolute lamination, linear channels fo med by current scour, rare large-scale cross-bedding, and some clast imbrication, show north-to-south flow. Continuous graded bedding in the sequence is rare, and grain orientations show that the sandstone beds themselves were deposited or redeposited by currents with the same orientation as those that cut basal flutings. Current structures, both internal and on bed soles, and perfection of graded bedding are inversely related. The conglomeratic mudflow deposits are noteworthy for a great variety of textural types, including pebbly mudstone, conglomerate with a dispersed framework and mud matrix, and other conglomerate with a sandy matrix or intact framework. The conglomerate beds are definite lateral equivalents of zones of failure up to 100 feet thick which include large synsedimentary contortions indicating mainly west-to-east slumping. The features of the zones indicate that they repres nt sea-floor deformation induced by the catastrophic introduction of the conglomeratic mudflows into the flysch environment. Geographic distribution, bed-thickness changes, and provenance of sandstone and conglomerate indicate original transverse sediment supply (normal or oblique to tectonic trends) into the flysch environment. In contrast, nearly all current structures indicate longitudinal distribution (parallel with tectonic trends). The deflection of gravity-controlled turbidity currents by the axial plunge of a geosynclinal trough could be indicated. However, the marked discordance between current and apparent slope directions over a wide area, the results of recent oceanographic research, and a general consideration of the paleogeography of flysch deposits with longitudinal paleocurrents suggest that an alternate working hypothesis be considered: downslope lateral supply by gravity-controlled mechani ms, including turbidity currents, sandflow, mudflow, or sliding, into a regime of marine bottom currents sufficiently powerful to distribute detritus and produce sedimentary structures. Sedimentary structures and paleontological evidence indicate that the dominant longitudinal currents in this example operated in both deep- and shallow-marine environments. Positive interpretation of either source or slope direction from current structures in flysch and flysch-like sequences is unwarranted without considerable supporting evidence.

Geological and Geophysical Studies in Western Part of Bengal Basin, India

Abstract: Just beyond the western boundary of West Bengal, the great Indian shield disappears below a blanket of alluvium. The exposed part of the shield bordering the Bengal basin is marked by a row of intracratonic Gondwana basins, a series of thrust zones in Singhbhum, and extensive exposure of basic volcanics in the Rajmahal Hills. Intensive geophysical surveys and deep drilling in the alluvium-covered plains of West Bengal have revealed a thick section of Cretaceous and Tertiary sediments lying on a basement of basalt lava flows, presumably of the same age as the Rajmahal Group volcanics. An extension of the easternmost Gondwana basin farther east, below the Bengal alluvium, also is suggested. A series of buried basement ridges, marking the western margin of the Bengal basin, resumably kept the Gondwana continental basins isolated from the main Bengal basin through most of Tertiary time. Locally, during the late Tertiary, the sea transgressed over these basement ridges and onlapped parts of the Indian shield. Flanking the eastern margin of the buried ridges is a row of basin-margin en echelon faults and scarps, possibly the shallower expressions of some deep-seated movements in the basement. East of this marginal fault zone lies the stable shelf of West Bengal with a homoclinal dip toward the southeast. Seven seismic reflectors mapped in the Mesozoic and Tertiary sediments of the shelf indicate uniform increase of thickness (3,000 to 27,000 feet, approximately) of these sediments toward the southeast. Except for a few normal faults, the area is practically undisturbed structurally. An extensive unconformity between the Miocene and Pliocene has been recognized. Locally, weak evidence of another depositional break, at the top of the Oligocene, is present. Around Calcutta, the Eocene key horizon (Sylhet Limestone) shows a conspicuous basinward flexure (the "hinge zone") at a depth of about 15,500 feet. East of this "hinge," which traverses the whole Bengal basin, lies the deeper part of the basin with a greater rate of subsidence and a different lithofacies. Seismic interpretation suggests a sharp lithofacies change at this zone, from the Eocene nummulitic limestone of the stable shelf to a thick sequence of clay and shale in the deeper part of the basin. In the younger Tertiary sediments is a similar change of facies from the arenaceous sediments of the stable shelf to the dominantly argillaceous sediments downdip. Marine transgression on the West Bengal shelf occurred during the Late Cretaceous (locally), late Eocene (extensively), and Miocene (in the eastern parts only). Except for these periods of marine transgression, sedimentation took place under fresh-water, estuarine, or deltaic conditions. A summary of the tectonic and depositional history of the whole region, from the eastern margin of the Indian shield to the folded belt in Assam, is given in conclusion. This integrates the work done in West Bengal by the Indo-Stanvac Petroleum Project with that done in Assam by the Burmah Oil Company and its affiliates.

Oil and Geology in Cuanza Basin of Angola

Abstract: The Cuanza basin is in northwestern Angola on the Atlantic Coast of West Africa. This basin is about 300 km. long north-south and 170 km. wide east-west, and contains an Early Cretaceous carbonate-evaporite sequence and a Late Cretaceous and Tertiary argillaceous-arenaceous sequence. The Precambrian crystalline basement is partly covered by extrusive rocks and granite-wash type sediments. Surface and subsurface sediments of the basin consist of Lower and Upper Cretaceous, Paleocene, Eocene, and Miocene strata. Occurrences of oil and gas have been reported in almost all of the stratigraphic units in the Cuanza basin, and there is major production from the Cretaceous rocks. Study of these hydrocarbon occurrences and of the geological history of the basin shows that close relationships exist between sources, migration, and entrapment of oil, and environment of deposition controlled by the basement and salt tectonics. During Early Cretaceous time, subsidence of the central part of a restricted basin determined the regional cyclical deposition of a carbonate-evaporite sequence providing a favorable situation for genesis and entrapment of oil. Thus, the deposition during Aptian time of a very fine crystalline limestone, interbedded with argillaceous limestone and overlain by an oolitic sandy calcarenite, itself underlying evaporites, had an important influence on the subsequent extent of oil accumulations in the Binga Formation. During Aptian-Albian time, differential subsidence on the western margin of the basin caused lateral interfingering of back-reef calcarenite, argillaceous carbonate, and evaporite. This interfingering is believed to be related closely to oil accumulations in this area. Very i portant vertical development of reef deposits in the Longa area is related to lateral migration of the underlying Massive Salt, which flowed with the help of the excess of weight introduced by the growing reef. On the eastern margin, upper Albian reef buildups capped by marine shale also provided a favorable situation for generation and accumulation of oil. During Late Cretaceous and Tertiary time, a major basement flexure or fault zone appears to have been associated genetically downdip with deposits that accumulated with greater thickness than elsewhere. This flexure and the loci of maximum deposition moved eastward during Late Cretaceous and Paleocene, then westward during Eocene and Miocene. These thick formations, which are mainly argillaceous-arenaceous and which were deposited partly in deltaic and lagoonal environments, grade westward into thinner marine deposits and eastward into thinner continental deposits. During each particular epoch corresponding with a stabilization of this moving flexure, favorable conditions for genesis of hydrocarbons seem to be related to these transitional environments. Oil production is located above the Massive Salt at the crest of salt anticlines, and one small oil field has been discovered below the Massive Salt along a ridge of the Basement Complex in a pinch-out of sandstones between Precambrian mica-schist below, and salt above.

Salt Structures of Gulf of Mexico Basin--A Review

Abstract: More than 300 diapiric structures formed by the intrusion of relatively pure salt are known in Alabama, Mississippi, Louisiana, Arkansas, Texas, Nuevo Leon, Veracruz, Tabasco, and Cuba. In form, the structures are rod-like, domal, anticlinal, and ridge-like. They rise vertically, or nearly so, and increase or decrease with height. Many are capped by residual masses of anhydrite, altered in varied degrees to gypsum, sulfur, and calcite. Modern theory postulates growth resulting from density differences between the salt and surrounding sediments (1) by upward movement of the salt through the overlying sediments in response to gravitational inequilibrium, or (2) by salt structures remaining at an essentially constant level while the surrounding sediments of sedimentary rocks moved downward around them as deposition progressed. Model studies suggest that variations in overburden and faulting are primary causes of the initiation of salt movement. The probable source of the salt in Gulf Coast salt domes is the Louann Salt. It may have been as much as 5,000 ft thick and have had an original volume of 200,000 cu mi. Sediments enclosing salt stocks have varied structural configurations. The strata may be arched, they may be ruptured and pierced by the salt, they may be complexly faulted, or they may be deformed by various combinations of faulting and folding. All the salt structures in the Gulf of Mexico basin probably are of similar genesis.

Seismic-Profiler Survey of Blake Plateau

Abstract: Continuous seismic-reflection profiles on the Blake Plateau are correlated with sediment cores, seismic-refraction data, and logs of wells on Florida to study the structural relations of the Florida peninsula and continental shelf with the plateau. Results indicate that the entire Blake-Bahama-Florida area was the site of shallow-water carbonate deposition behind a barrier reef until late in the Mesozoic. Death of the reef along the Blake Plateau margin and continued subsidence of the entire area created the present submerged plateau. Florida and the Bahama banks continued to build throughout the Tertiary and have maintained an elevation near sea-level. Four strong reflectors are observed in the plateau sediments, the deepest apparently representing an interface within the Upper Cretaceous and correlating with the top of a 4.5 km./sec. refracting layer. The overlying beds probably are composed of bank-derived calcarenite and calcilutite; the reflectors correspond to major changes in sediment types or rates of deposition. The Tertiary sediments on the plateau essentially form a wedge, about 1,200 m. thick on the west side and 200-300 m. thick near the escarpment on the east. The extension of the Cape Fear arch southward onto the plateau apparently diverts and restricts the flow of the Gulf Stream. This has resulted in extensive erosion of much of the surface of the plateau. In places the erosion has uncovered beds of Eocene and Paleocene age. The material eroded from the plateau by the stream, as well as that which stayed in suspension, has b een swept off the plateau and has formed the Blake-Bahama outer ridge.

Paleoecology and Diagenesis of Key Largo Limestone, Florida

Abstract: The Key Largo Limestone is a Pleistocene reef limestone that forms the upper Florida Keys. It consists of an organic framework of coral colonies and an interstitial skeletal calcarenite, both of which are homogeneous throughout the upper part of the formation. The Pleistocene reef was formed chiefly by massive and rounded coral heads, the principal frame builder being Montastrea annularis. No organic zonation is visible perpendicular to the long axis of the reef. Both a well-sorted and a poorly sorted calcarenite facies can be distinguished. The poorly sorted facies, which predominates, surrounds coral heads and is characterized by its high calcilutite content. The well-sorted facies forms channel deposits between coral buildups. In organic composition, the ancient Key Largo reef is roughly equivalent to the deep-water Montastrea annularis zones of living West Indian bank reefs and to West Indian patch reefs. It grew under semi-restricted conditions, possibly as a series of coalescing patch reefs, but more probably below the surf zone in op n water, where it spread laterally over large areas of the sea floor. The constituent composition of the two calcarenite facies indicates that aragonite and high-magnesium calcite originally were the chief mineralogic components. Fossils that initially were high-magnesium calcite have been altered in place to low-magnesium calcite through the removal of excess magnesium by fresh water in the meteoric zone; such fossils have otherwise remained intact. Aragonit, on the other hand, currently is being dissolved in the meteoric zone, and the dissolved CaCO3 is being redeposited near the site of dissolution as low-magnesium calcite cement. Many fossils that originally were aragonite now remain only as molds of sparry calcite. The rate of aragonite dissolution and calcite cementation is accelerated by artificial exposure. The diagenetic fabrics exhi ited by the Key Largo Limestone should be useful in recognizing ancient limestone units that were subjected to subaerial exposure soon after deposition.

Seismic-Refraction Study of Continental Margin East of Florida

Abstract: Data from 31 seismic-refraction profiles are interpreted and presented in five structure sections. The subsurface structure east of Florida under the Blake Plateau is similar to that of the margin north of Cape Hatteras. Basement plunges seaward from Florida into a deep sedimentary trough under the Blake Plateau. A basement ridge parallels this north-south-trending trough on the seaward side, along the eastern edge of the plateau. The basin under the Blake Plateau was separated from the South Florida-Andros Island basin, at least until the Early Cretaceous, by a southeastward extension of the Peninsular arch. This seaward extension trends from just east of Cape Kennedy to the western end of Little Bahama Bank. The marked relief of the Florida shelf, the Florida Straits, and the Blake Plateau is evident only in the sedimentary layers and is the result of significant changes in thickness of the post-Paleocene section, especially the Eocene. The top of the Paleocene extends beneath the present physiographic irregularities with slight relief, whereas the Eocene thickness ranges from about 500 m. on Florida to about 70 m. on the Blake Plateau. Strong currents sweeping the Florida Straits and Blake Plateau probably are responsible for the absence of a thick post-Paleocene section The present orientation of the Florida Current may have existed as early as Paleocene time. Besides the changes in relief within the sedimentary section, there are lateral facies changes and velocity variations. These variations are primarily dependent on the depth of burial. Nearly similar velocity-depth distributions are found for the Blake Plateau, the Florida Straits, the Bahama banks, and the Florida platform. The variations in thickness of the sediments in these areas result in variable velocities in the stratigraphic units, and correlation of the refraction data is difficult.

Pre-Pennsylvanian Paleotectonics--Key to Basin Evolution and Petroleum Occurrences in Paradox Basin, Utah and Colorado

Abstract: A large northwest-trending graben-faulted anticline consisting of late Precambrian through Mississippian rocks is exposed in the core of the San Juan Mountains near Silverton, Colorado. The structure was formed before Late Cambrian time when younger Precambrian quartzite beds were down-faulted extensively into the older Precambrian basement complex. The structure stood as a high topographic feature because of differential erosion during Late Cambrian to Late Devonian time, but was largely buried by the Late Devonian upper Elbert Formation. Renewed activity occurred in latest Devonian or earliest Mississippian time, when tidal flats developed on the high flanks of the fault block while normal marine waters moved into the graben. The entire structure was high during the Mis issippian, because the Leadville Formation is preserved only as tidal-flat dolomite and weathered residual blocks within the overlying regolithic Molas Formation. Pennsylvanian and later fault movement occurred, as Pennsylvanian Hermosa and Permian Cutler strata were displaced progressively in the graben. The use of this paleotectonic feature as a model makes other areas more readily understood. A similar ancient fault block south of Ouray, Colorado, extends northwest into the subsurface of the eastern Paradox basin. This structure joins a major northwest-trending pre-Pennsylvanian system of faults along each of the major salt anticlines, which are parallel with the adjacent Uncompahgre uplift. Isopachous and lithofacies studies reveal that these structural lineaments already were present in Late Cambrian time, and that they actively controlled sedimentation through Mississippian time. It is possible that the closely related Uncompahgre uplift had a similar early history. Pre-Pennsylvanian reservoir facies are best developed high on the upthrown fault blocks. Late Devonian linear sand bars and Early Mississippian biogenic crinoid banks, which are associated with all production from the Devonian (McCracken) and Mississippian (Leadville), formed in the shallow-water environments that developed along these structural flanks. However, where paleotectonic relief was too great, pre-Pennsylvanian rocks are missing either because of non-deposition or subsequent erosion. The downfaulted paleotectonic troughs were the site of thick salt deposition during Pennsylvanian time. When Middle Pennsylvanian to Early Permian clastic wedges from the Uncompahgre uplift apparently triggered salt flowage by differential loading, the fault blocks probably acted as buttresses that deflected the plastic salt upward. Consequently, the salt anticlines grew along the linear trends created by the faults.

Paleogeomorphology and its Application to Exploration for Oil and Gas (With Examples From Western Canada)

Abstract: Under the term paleogeomorphology are grouped all geomorphological phenomena which are recognizable in the subsurface. Buried relief features with a marked three-dimensional geometry are of importance to the petroleum geologist whenever they lead to the trapping of hydrocarbons. Paleogeomorphic traps form a third and distinct group which ranks in importance with stratigraphic and structural traps as a major mechanism for localizing hydrocarbon occurrences. They are not simply another type of stratigraphic trap, but form by themselves a category of considerable economic significance. Paleogeomorphic traps can not be analyzed, nor can their occurrence be predicted, by stratigraphic or structural methods of study, but must be treated as a geomorphological problem. Of the man types of buried relief features, some are of greater interest to the petroleum geologist than others. In this paper, the morphology of buried erosional landscapes is discussed in greater detail. Other types of buried relief features are fossil reefs, barrier beaches, and submarine canyons. Hydrocarbons may be trapped either directly or indirectly as a result of paleogeomorphological processes. In either case, the traps may occur below or above (against) the morphological surface. Numerous examples of the effects of erosion are known from the Paleozoic and Mesozoic buried landscapes of western Canada and the Mid-Continent area of the United States. Hydrocarbon traps occur below the highs on such erosional surfaces as well as in sandstone bodies deposited in the lows on these surfaces. The rules which govern the formation of ancient landscape forms are worked out in detail, with particular emphasis on the application of quantitative geomorphology to the pattern of the ancient drainage system and on such features as summit levels and the influence of geological factors (er sion-resistant levels, influence of faults and fractures, etc.). Weathering and underground solution also play a role in providing both reservoirs and buried traps for oil and gas. The method of analysis must take into account (1) the structural attitude of the strata below and above the erosional surface, (2) the lithology of the formation overlying the unconformity (if used for isopachous mapping), (3, 4) synsedimentary structural movements and the compaction factor in the formation overlying the unconformity, (5) problems of correlation, (6) reservoir development, (7) presence of a "seat seal," and (8) other factors. Paleogeomorphology provides an interesting evaluation of "modern" geomorphological thinking. The fossil relief forms "frozen" by the transgression of younger rocks neither disprove the validity of the classical peneplain concept, nor do they support fully the newer ideas regarding slope retreat. The actual conditions found are described by a re-definition of the old term paleoplain. The summit level is an intrinsic part of this feature, and not a dissected earlier peneplain. A new geomorphological concept introduced here concerns the alternation of obsequent and resequent interfluve spurs. Buried landscapes should provide a high percentage of the future oil and gas fields yet to be discovered in North America. The most important stratigraphic levels at which buried landscapes occur are those that formed after major periods of orogenesis. Their geographical locus corresponds to the broad belts of subsequent transgressions.

New Seismic Data Concerning Sediments and Diapiric Structures in Sigsbee Deep and Upper Continental Slope, Gulf of Mexico

Abstract: Emerged knolls and buried domes in the Sigsbee Deep and on the upper continental slope of the Gulf of Mexico were recorded on an Electro-Sonic Profiler (ESP) and Precision Depth Recorder (PDR) survey from Galveston to the Campeche Bank and back. Numerous diapiric structures were observed north of the Sigsbee Scarp, suggesting possible continuity of a belt of such structures extending from the Texas-Louisiana coast to the scarp. The scarp appears to be the front of the system of diapirs. The diapirs parallel with the scarp seem to be ridges that may have played a role in the building of the continental borderland by retaining sediment moving from the continent toward the basin. The Campeche continental shelf in the area surveyed appears to be typical of a depositional terr ce resulting from upbuilding and outbuilding; there is little or no evidence of reef growth. The previous finding that some of the shallower Sigsbee domes are indicated by topographic highs of a few fathoms of relief was substantiated; these highs were shown to be underlain by some PDR sub-bottom reflectors having an even greater relief than the topographic highs.

Deep Channels in Turbidite-Bearing Formations

Abstract: Channels from 3-20 m. deep occur in three turbidite-bearing formations in England, the Shale Grit and Grindslow Shales (Upper Carboniferous, North Derbyshire) and the Westward Ho! Formation (Upper Carboniferous, North Devon). Although most channels in the Shale Grit and Grindslow Shales are single cuts, some show several related phases of cutting and filling. The walls generally have gentle slopes, but in one channel they slope 70°. The channel-fill sediments consist of thick sandstone beds interpreted to be proximal turbidites. The Westward Ho! Formation has two wide, shallow-walled, turbidite-filled channels, and three mudstone-filled channels in which both the pre-channel sediments and the fill are of the same facies. Based on studies of the processes of channel erosion by permanent ocean currents, tidal currents, turbidity currents, catastrophic currents, rivers of sand, and sand creep, it is concluded that the most likely process for the erosion of the deep channels in the Shale Grit and Grindslow Shales was by fast underladen turbidity currents. In the Westward Ho! Formation some channels could have been cut either by underladen turbidity currents or by permanent ocean currents, but there is insufficient evidence to permit a more definite interpretation. The other Westward Ho! Formation "channels" are believed to be slump scars. Processes of channel fill are: deposition from traction currents, sand creep, and turbidity currents; the last is considered to be the most likely process in all three f rmations. Erosion by underladen turbidity currents usually takes place only at the up-current edge of the apron of turbidites in a basin. The presence of deep channels in turbidite-bearing formations therefore has paleogeographic significance.

Dispersal and Petrology of Sandstones of Stanley-Jackfork Boundary, Ouachita Fold Belt, Arkansas and Oklahoma

Abstract: Sandstones of the uppermost Stanley and lowermost Jackfork Groups (both flysch deposits) include arkose, subarkose, subgraywacke, feldspathic subgraywacke, and orthoquartzite (Folk, 1954, sandstone classification). Regional study of the uppermost part of the Stanley Group (Moyers Formation) shows that arkose and subarkose occur in the western Ouachita Mountains, and arkose and feldspathic subgraywacke occur in the south-central Ouachitas. Orthoquartzites occur only in the northeastern part of the fold belt. In the lowermost part of the Jackfork Group (Wildhorse Mountain Formation), subarkose occurs in the southern part of the Ouachita geosyncline, whereas subgraywacke is limited to the northwestern part of the geosyncline. Orthoquartzite occurs throughout the rest of the uachita fold belt. A linear regression analysis comparing feldspar content with both clay matrix and authigenic mica indicated that the amount of diagenetic matrix is negligible. Most of the authigenic mica appears to be recrystallized primary matrix. Geographic distribution of sandstone classes suggests that the Moyers Formation was derived from two sources. Quartzose material was derived from a northern source (Ozarks) and feldspathic material was derived from a southern source. Detritus was transported laterally into the Ouachita geosyncline. The sandstone of the Wildhorse Mountain Formation was derived from the same two sources and from a third source east of the present outcrop belt. Quartzose sandstone appears to have been deposited by a combination of lateral supply from the Ozarks and Illinois basin and subsequent redistribution by axial currents. Lateral supply was accomplished by turbidity currents, sand flow, and submarine slumping. Directional mapping of flute casts by others indicated that deposition of sand in the Ouachita geosyncline was accomplished by axially oriented currents rather than by downslope currents. The assumption that axially oriented flute casts were formed by the same turbidity currents that transported sand from the geosynclinal margin is inconsistent with interpretations based on mineralogical data. Oceanographic studies show that deep-water currents, flowing parallel with topographic strike, flow at velocities sufficiently high to erode silt and clay. Similar ocean currents probably reworked the marginally supplied sands and resedimented them in an axial direction in the Ouachita geosyncline. The axial orientation of some sole marks in ancient flysch deposits possibly records the direction of flow of ancient resedimenting ocean currents, whereas the regional distribution of sandstone types possibly indicates lateral supplying of sand by gravity-induced turbidity currents, submarine slumps, or sand flow. An oceanographic model consisting of sand emplacement by gravity-induced processes and later resedimentation by axially oriented ocean currents is offered as the most plausible mechanism to resolve anomalous provenance interpretations based on mineralogical and directional data.

Patterns of Chemical Composition in Deep Subsurface Waters: GEOLOGICAL NOTES

Abstract: Stagnant connate waters associated with oil fields are similar in ratios of ions and cations. The principal ion is chloride--nearly always 99 percent. The principal cations are sodium, calcium and magnesium, and the usual Ca/Mg ratio is 5 to 1 (in equivalent weights). The concentration ranges from about 50,000 to 350,000 ppm. There is a general tendency for this type of water to increase in concentration with depth. In artesian situations the compositions of oil field waters is different. They range in concentration from 5,000 to 15,000 ppm. Sodium is the predominant cation, 85 to 100 percent and calcium and magnesium are rare or absent. These waters are classified into two types by their anions: (1) those without SO 4 and HCO 3 from 3 to 85 percent and (2) those with more than 50 percent SO 4 .

Stratigraphic Traps in a Valley Fill, Western Nebraska

Abstract: Oil is trapped in a trend of valley-fill sandstone bodies in the Cretaceous "J" interval in Cheyenne and Morrill Counties, Nebraska. The valley fill is composed chiefly of porous and permeable sandstone; it trends north-south and is about 1,500 ft. wide and 50 ft. thick. Oil has accumulated in the valley fill where it crosses the axes of northwest-plunging anticlines. Updip (eastward) escape of oil is prevented by the enclosing marine sediments of the "J" interval, in which sandstone beds with low oil-entry pressures are discontinuous and separated by sandstone or shale beds with higher oil-entry pressures. The traps therefore involve a combination of stratigraphic and structural conditions. The "J" in this area is a sandstone and siltstone unit, 38-77 ft. thick, deposited in predominantly marine environments. The "J" is overlain and underlain by dark gray marine shale. The interval can be divided into two marine members, an upper and a lower, each relatively thin and with distinctive mineralogy, sedimentary structures, fossil content, and electric-log character. These members can be traced over hundreds of square miles in western Nebraska. After deposition of the upper member, the area emerged and a stream cut a narrow valley which was filled mainly with sandstone of distinctive character. The stream in the valley cut through most or all of the two previously deposited marine members. The trend of the valley fill is nearly straight, suggesting that erosion and deposition were the work of a meandering stream whose width was much less than that of the valley. Seven fields have been discovered along the valley-fill trend within the study area. One well of every 1.9 wells drilled into the valley fill has been completed successfully. These wells are rated as good producers and have long productive lives by Denver basin standards. Some production also has been developed in marine sandstone beds of the "J" near the area of the valley fill, but only one well of approximately every 10.9 drilled is completed successfully; moreover, productivity commonly is low and total reserves are small. Therefore, stratigraphic study leading to an improved understanding of the genesis and form of the sandstone reservoirs is of considerable economic value.

Time-Stratigraphic Analysis and Petroleum Accumulations, Patrick Draw Field, Sweetwater County, Wyoming

Abstract: The search for new petroleum reserves can be implemented greatly by a more thorough understanding of why petroleum is trapped where it is. The Patrick Draw field, discovered in 1959, started a wave of exploratory effort in the Rocky Mountain area to find additional giant stratigraphic traps in the Upper Cretaceous rocks where porous and permeable sandstone pinches out on structural noses. The lack of success in finding another "Patrick Draw," despite this widespread exploratory effort, means that factors not generally considered must have had a dominant influence in the accumulation. These factors are revealed only by reconstructing the geologic history of the area, beginning with the deposition of the reservoir and source rocks and tracing the structural attitude of thes rocks through time. Although several sandstone reservoirs produce petroleum at Patrick Draw, the principal productive interval consists of two sandstone bars at the top of the Almond Formation (Upper Cretaceous). The spatial dimensions, lithologic character, and stratigraphic framework of these bars suggest that they are barrier-bar sandstone bodies deposited along the margin of the Lewis sea. These porous and permeable linear barrier bars have a general north-south trend and grade updip on the west into impermeable shale and sandstone that were deposited in a swamp and lagoonal environment. A second important productive interval is approximately 40 ft. below the top of the Almond Formation. The areal distribution, lithologic character, and stratigraphic framework of sandstone in this interval suggest th t it was deposited as a tidal delta in a lagoon. Each of the three main productive sandstone bodies has a different oil-water contact. The geologic history of the Patrick Draw area shows that, by the beginning of deposition of the Lance Formation (Upper Cretaceous), conditions were favorable for petroleum accumulation. The reservoir sandstone beds had 1,200 ft. of overburden which had accumulated in the several million years since the reservoir sandstone beds were deposited. An early trap was formed where these sandstone beds were warped over an east-plunging structural nose, and early migration of petroleum produced a large accumulation a few miles south of the present field. When the present Wamsutter arch came into existence in post-early Eocene time, the old trap was opened and the accumulation spilled northward to be trapped at the present location of the Patrick Draw field. The search for more "Patrick Draws" must include more than an analysis of present structure and potential reservoir rock. The time of formation of the trap, structural modification of the trap through time, and associated origin and migration problems are hidden factors that play the dominant roles in the formation of a large petroleum accumulation. Exploration geologists must learn more about the regional framework of sedimentation--the cause and effect of incipient structural development in depositional areas. They must understand how these factors relate to the geologic history of a region.

Late Mesozoic Stratigraphy and Tectonic History, Port Orford-Gold Beach Area, Southwestern Oregon Coast

Abstract: During Nevadan orogenesis (ca. 145 m.y. ago), the Galice(?) Formation was regionally metamorphosed (greenschist facies) to phyllites and schists of the Colebrooke Formation, the Pearse Peak Diorite was emplaced, and the Port Orford-Gold Beach area, Oregon, underwent intense compressional deformation. The ancestral Klamath Mountains and Sierra Nevada became important source areas for sediments. Eugeosynclinal conditions in coastal Oregon, however, persisted into latest Jurassic (Tithonian) time. The uppermost Jurassic Otter Point Formation (new name) includes volcanic rocks, radiolarian cherts, and many graded beds--lithologic types that are unknown in the correlative Riddle Formation. Thus, the distribution of latest Jurassic facies in southwestern Oregon was comparable with that of the coastal Franciscan and the inland "Knoxville" Formations of northern California. Late Mesozoic eugeosynclinal conditions in southwestern Oregon were terminated by Diablan orogenesis at the end of the Jurassic (ca. 135 m.y. ago). Northwest-striking wrench faults and related folds were superimposed on the northeast-striking, Nevadan structural grain. Dioritic, gabbroic, and ultramafic bodies, including the Vondergreen Hill Peridotite (new name), were emplaced. Tectonic lands probably rose by differential movements related to wrench faulting. Coarse gravels of the Humbug Mountain Conglomerate (new name) and, later, progressively finer sediments of the Rocky Point Formation (new name) were shed into a transgressing and deepening Early Cretaceous (Valanginian) sea. Younger Cretaceous and Tertiary evolution of the Port Orford-Gold Beach area, although poorly documented, involved extensive fragmentation and folding of the Lower Cretaceous about north- to northeast-striking axes. Several post-Diablan orogenies affected this area, including widespread major diastrophism that began in middle and late Miocene time along the Pacific coast. The most recent phases of structural development were uplift of Pleistocene terrace deposits and faulting. Middle Miocene(?) deposits near Port Orford perhaps represent the southern limit of the Miocene within the coastal, westward-facing Tertiary embayment centered about Newport, Oregon. The Miocene has favorable reservoir characteristics and should be a prime target for offshore exploration.

Stratigraphic vs. Structural Controls on Carbonate-Mound Hydrocarbon Accumulation, Aneth Area, Paradox Basin

Abstract: Pennsylvanian oil and gas accumulations in the southern Paradox basin are in carbonate mounds of Desmoinesian age. Major oil production at Aneth, Ismay, Tohonadla, Gothic Mesa, Anido Creek, and other fields is from algal mounds elongate in a general northwest-southeast direction along the basin shelf. Carbonate reservoirs are associated closely with sapropelic black shale and evaporite, which occur in cyclic repetition in the shelf area and grade basinward to a predominantly salt section. All reservoirs appear to be isolated bodies of porous carbonate, mostly limestone. About 30 oil and gas fields productive from the Pennsylvanian have been found in the Four Corners area; approximately half are classified as stratigraphic and the other half as either structural or structural-stratigraphic. In almost all fields, it can be demonstrated that the accumulation would have formed even if no structural closure were present, although in many fields the oil is localized by subsequent structural growth. The Aneth, Ismay, and Cache fields are primary examples of Paradox basin fields showing major stratigraphic and only minor structural influence on accumulation.

Geophysical Anomalies Over Precambrian Rocks, Northwestern Uncompahgre Plateau, Utah and Colorado

Abstract: Precambrian rocks exposed in the core of the northwestern Uncompahgre Plateau, Utah and Colorado, may be subdivided into four main units, each of which has distinctive lithologic or geophysical properties: (1) an oldest sequence of complexly folded biotite and feldspathic gneiss, amphibole gneiss, and amphibolite, exposed in Westwater Canyon of the Colorado River and tributaries; (2) gneissic granodiorite, exposed along Coates Creek and the upper Little Dolores River; (3) a batholith of coarse porphyritic biotite quartz monzonite, which extends from Ryan Park, Colorado, northwest to Spring Canyon, Utah; and (4) a pluton of metagabbro or metadiorite, 2 miles in diameter, exposed in the lower canyon of the Little Dolores River and in Marble Canyon. The quartz monzonite and etagabbro intrude the older gneissic sequence. The quartz monzonite and metagabbro are relatively magnetic and cause positive aeromagnetic anomalies; the metagabbro is relatively dense and causes a local gravity high. The gneiss and amphibolite are relatively non-magnetic, and their densities exhibit wide variations. Regional gravity values increase by 50 mgals. from Sagers Wash syncline, in the Paradox basin, to the crest of the Uncompahgre Plateau. This large anomaly is attributed to three combined factors: (1) structural relief of about 16,000-19,000 feet from the Precambrian surface in the Paradox basin to the crest of the plateau; (2) the northeastward wedge-out of about 12,000 feet of Paleozoic sedimentary rocks, including several thousand feet of Pennsylvanian evaporites, against the paleotectonic highland that was ancestral to the present Uncompahgre Plateau; and (3) density variation of Precambrian rocks, which are relatively more dense near the crest of the plateau than they are on the southwestern flank. Juxtaposition of the relatively magnetic quartz monzonite and the non-magnetic sedi entary rocks along the front of the uplift causes an aeromagnetic high of about 500 gammas. The metagabbro pluton gives rise to a residual gravity high of 6-8 mgals. and an aeromagnetic high of 300 gammas. The sequence of gneiss and amphibolite is characterized by gentle magnetic gradients and low-amplitude anomalies. On the basis of the geophysical anomalies, quartz monzonite is inferred to extend beneath the cover of Mesozoic rocks from Spring Canyon northwest to Cisco, Utah; a concealed mafic pluton, about 2 miles in diameter, is inferred in T. 20 and 21 S., R. 24 E., northeast of Cisco; and a belt of quartz monzonite is inferred to extend northwest from T. 11 S., R. 104 W., Colorado, to T. 19 S., R. 25 E., near Harley Dome, Utah. The remaining areas of the northwestern part of the plateau are inferred to be underlain by relatively non-magnetic gneiss and amphibolite.

Classification of Reservoir Rocks by Surface Texture

Abstract: Geologists exploring for oil and gas need quantitative field criteria for estimating the reservoir potential of rocks. Such criteria are presented in this paper. They were derived by correlating visual rock characteristics with measured reservoir properties. The criteria are based on approximately 2,000 samples of various lithologic types from several areas. The rocks were first examined in polished section with the binocular microscope and classified according to surface texture. They then were analyzed for reservoir properties of porosity, permeability, and pore-size distribution. An empirical correlation was found between these rock textures and reservoir properties. A classification was developed in which both sandstone and carbonates were divided into four major surface-textural types. Using the empirical correlation that was found, one can estimate reservoir properties from surface texture alone.

Frio Barrier Bar System of South Texas

Abstract: A study of available subsurface data from wells drilled in the South Texas Counties of Aransas, Nueces, Refugio, and San Patricio discloses the presence of a barrier bar system in the Frio formation (Oligocene in age) which is easily divisible into three distinct depositional environments. With the termination of the major Vicksburg transgression, the seas began their slow withdrawal and deposition of Frio clastics commenced. Within this regressive framework of the Frio, a massive sand bar developed which is analogous to the present day Padre-Mustang-St. Joseph-Matagorda Island complex. These Frio bar sands were transported by longshore currents from an area to the southwest where extensive deltas were being built by the ancestral Rio Grande River. Within the Frio bar system continental shelf, bar, and lagoonal environments are recognizable. Prolific production has been established in sands occurring in each of these environments. The success of future exploration within the Frio is largely contingent upon a thorough understanding of the depositional framework of this Frio barrier bar system.

Stratigraphy and Classification of Type Esna Group of Egypt

Abstract: The Esna Shale of the Esna-Idfu region, Nile Valley, Upper Egypt, long has been treated as a single formation. Lithological and paleontological data justify raising the Esna to group status and subdividing it into two formations, the Sharawna Shale (Maestrichtian), below, and the Owaina Shale (Paleocene), above. The two formations are separated by a thin conglomerate and have distinctive lithologic characteristics and different faunas. The base of the Esna Group, as redefined, is the top of the Sibaiya (phosphate) Formation (Senonian). The top is the base of the Thebes calcareous shale member of the Thebes Formation (Eocene). The Thebes calcareous shale member usually has been included with the Esna, but is excluded here on paleontological-lithological grounds.

The Obscure and Subtle Trap

Abstract: The tremendous expanding demand for petroleum and its products that continues to develop means that we must take a hard look at our future sources of petroleum supply. In spite of the fact that most exploration has been and is directed toward the search for petroleum in local structural traps, many of the largest oil and gas pools in the Western Hemisphere are trapped by non-structural phenomena. Structural traps are so obvious that they are the first to be tested, but we are now facing a situation in which the supply of structural traps in the United States seems to be limited; untested anticlines are becoming more difficult to find. Does this indicate an impending shortage of petroleum? The answer would seem to be no--but this means that the search will have to be for m re obscure and subtle trapping situations. The search will continue for the purely structural trap, out there will be added stratigraphic variations and fluid-flow phenomena, all operating either together or independently. We have "stumbled" into many great non-structural oil and gas pools while looking for purely structural traps, but the time seems to have arrived when we must start looking directly for combination traps of all kinds involving different proportions of structure, stratigraphic change, and fluid-flow phenomena. Such traps may contain very large petroleum pools, as past experience has shown. There are in the Rocky Mountain region many such untested potential combinations of large structures, stratigraphic changes, and favorable fluid-flow conditions to justify the belief that this region has a continued great future as a petroleum-producing region of importance to our national needs. The fact that the Rocky Mountain Section of the Association is dedicating a full meeting to the obscure and subtle trap is a sure indication of a change in our thinking. Once we start actively to look for traps that combine structure, stratigraphic change, and fluid phenomena instead of to look only for local structure, there is no reason why discoveries in the United States should not continue to meet the demand. The Rocky Mountain region has as bright a future for petroleum discovery as any other.

Strata of Late Precambrian-Cambrian Age, Death Valley Region, California-Nevada

Abstract: The Noonday Dolomite, Johnnie Formation, Stirling Quartzite, Wood Canyon Formation, and Zabriskie Quartzite, which seem best assigned to the late Precambrian-Cambrian interval, are a conformable and predominantly detrital part of the stratigraphic record of the Death Valley region. They can be traced from type localities in the Spring Mountains and in the well-known section in the Nopah Range southwestward to the Salt Spring Hills and Silurian Hills in southern Death Valley and southeastward to the Kingston Range-Winters Pass area. Satisfactory correlation of these widespread formations is facilitated by (1) regional persistence of most of the members as originally recognized in the Nopah Range, (2) the distinctive lithologic character of the Noonday Dolomite, and (3) the presence of two marker units--an oolite and a volcanic ash(?)--high in the Johnnie Formation. Isopachous contouring of the stratigraphic interval from the top of the Zibriskie Quartzite to the base of the Noonday Dolomite appears to outline a late Precambrian-Cambrian marine trough. The major axis of this trough strikes north-northwest and is approximately coincident with the present Amargosa Valley and southern Death Valley. Along this axis, the combined thickness of these strata or their apparent equivalents decreases progressively from nearly 12,000 feet in the Funeral Mountains to only a few hundred feet in the Marble Mountains 160 miles toward the south. The isopachous lines drawn near the southeastward bend in the Garlock fault zone are nearly parallel with this major break, and the projected zero contour is about 10 miles southwest of it. This pattern strongly suggests that the structural element south and southwest of the Garlock fault, generally referred to as the Mojave block, existed as a topographic high in late Precambrian-Cambrian time.

Regional Stratigraphy of Frontier Formation and Relation to Salt Creek Field, Wyoming

Abstract: About 420,000,000 barrels of oil have been produced from Salt Creek field, Natrona County, Wyoming. Most of this production is from the second Frontier sandstone, which is one of many sandstone bodies interbedded with marine shale in the lower part of the Upper Cretaceous in the Rocky Mountain area. The stratigraphic interval between the Mowry Shale and the Niobrara Formation contains the Frontier and equivalent formations. The interval containing this section is more than 1,000 feet thick in central, northeastern, and west-central Wyoming and in southeastern Montana. Another area of thick sediments within this interval is in northwestern Montana and western Alberta. In some areas this section is entirely marine shale; in other areas it contains abundant sandstone bodies. The sand was transported by a series of river systems that formed deltaic complexes or lobate sand concentrations at several places along the margins of the early Late Cretaceous sea. These deltaic deposits are represented by the "D" sandstone of the Denver basin, the Ferron Sandstone Member of the Mancos Shale of Utah, the Cardium and Bad Heart Sandstones of Canada, and the Frontier Formation of Wyoming. The second Frontier sandstone that produces oil at Salt Creek field is an offshore bar associated with the eastern terminus of one stage of Frontier deposition. The sandstone body is several miles wide, more than 60 miles long, and as thick as 100 feet. Salt Creek anticline (formed in late Late Cretaceous or early Tertiary time) is in an area of excellent sand development where stratigraphic traps were developed before secondary migration of oil into present structural positions took place. There are other sandstone bodies included in the Frontier depositional complex which contain stratigraphically trapped oil, but are not draped over an obvious anticline. The Wind River and Big Horn basins and parts of the Green River and Powder River basins probably contain more "Salt Creeks"!

Evidence of Unconformity at Top of Trenton Limestone in Indiana and Adjacent States

Abstract: Subsurface data indicate that in Indiana and parts of adjacent states the Trenton Limestone was exposed to subaerial erosion before the deposition of the overlying shale, which generally is considered to be Cincinnatian in age. The subsurface data include: possible truncation of the Trenton Limestone in southern Indiana; knife-sharp contact between the Trenton Limestone and the overlying shale; concentration of pyrite, angular chert, phosphatic grains, and brecciated limestone and dolomite; solution pits on the surface of the Trenton Limestone and solution cavities within the Trenton Limestone; and local relief on the surface of the Trenton Limestone. East of a line, here called the hinge line, which extends from the Findlay arch to the eastern edge of the Ozark dome, the writer has not observed evidence of unconformity.

Petrography of a Cayugan (Silurian) Stromatolite Mound and Associated Facies, Ohio

Abstract: This algal stromatolite mound and associated facies are located along the southeast margin of the Michigan basin, on the Niagaran reef barrier that isolated the basin during the Cayugan (Salina sedimentation) Epoch. A petrographic study of 143 samples has shown eight major related microfacies which constitute the mound and associated facies: hemispherical stromatolite-constructed dolomite, both well- and poorly laminated; stromatolitic dolobreccia; flat stromatolite-constructed dolomite; grain-supported pelletoidal dolarenite; evaporitic dolosiltite; intraformational micro-dolobreccia; and grain-supported worm-tube dolarenite. These microfacies form five major depositional units. At the bottom of the stratigraphic section is the hemispherical stromatolite-constructed dolomite unit which forms the mound structure. The mound is overlain by the stromatolitic dolobreccia unit, which consists mainly of brecciated material similar to that forming the mound. This is overlain in turn by the flat stromatolite-constructed dolomite unit, the pelletoidal dolarenite unit, and the evaporitic dolosiltite unit, which is the typical Greenfield Dolomite. The environments represented by these units range from high intertidal or supratidal to shallow subtidal. The entire sequence is related to generally quiet, restricted, and penesaline conditions. Comparison with several Recent carbonate environment analogs, and use of Walther's Rule, suggest that a general transgressive phase of the five laterally juxtaposed environments would build the stratigraphic section described.

Stratigraphy and Deformation of Paleozoic Section at Anaktuvuk Pass, Central Brooks Range, Alaska

Abstract: The stratigraphic section at Anaktuvuk Pass consists of more than 13,000 feet of marine and non-marine sedimentary rocks ranging in age from Late Devonian through Permian. Non-marine Upper Devonian rocks include more than 6,300 feet of plant-bearing shale, siltstone, sandstone, and conglomerate, overlying a thick sequence of shale and slate. The Mississippian section, consisting of 4,680 feet of fossiliferous marine sandstone, black shale, dolostone, limestone, and chert, is overlain unconformably by approximately 400 feet of Permian marine shale and siltstone. Post-Paleozoic deformation of these rocks resulted in large-scale imbricate deep-seated thrust faults and large folds overturned toward the north; near the north front of the range the structures grade into broad o en folds associated with shallow thrusts that are limited to Mississippian and younger rocks. Most structural axes, both major and minor, trend east-west, and most thrusts, which tend to be localized along incompetent rock units, dip south, indicating northward release of deformational stresses. The Anaktuvuk thrust, exposed beneath isolated klippen on mountain summits northwest of Anaktuvuk Pass, is a major structural feature with minimum northward displacement of 8 miles. This thrust, confined to Mississippian and younger rocks, is shallow and appears to have moved northward from a structural high that forms the crest of the range. Absence of pre-Upper Devonian strata in exposed deep-seated thrust sheets suggests that the lowest unit of the Paleozoic section at Anaktuvuk Pass, thick in ompetent shale and slate, may constitute the base of a regional decollement in the northern half of the range. Within the area, intensity of deformation decreases gradationally northward as the Paleozoic section thins and as detrital sediments grade into coarser facies.