Basement Fault Blocks Research Articles

Geology of Norton Basin, Northern Bering Sea: ABSTRACT

The rocks that floor the Norton basin are most likely of Precambrian and Paleozoic age, like those that crop out around the basin. A maximum of 6.5 km of mainly Cenozoic rocks lies over basement in the basin. I believe that alluvial fans are present deep in the basin and border major basement fault blocks. These fans are the lowest units of the basin fill in many areas and probably consist of uppermost Cretaceous and Paleogene, coal- and volcanic-rich rocks. Mainly clastic nonmarine sedimentary rocks overlie the fan deposits. The Neogene and Quaternary basin rocks apparently were deposited in a marine environment. The Norton basin comprises two structural areas that are separated by a major northwest-striking horst. The first structural area lies west of this horst, where major normal faults strike northwest to form local areas where the basin is as deep as 5.0 km. The second area, east of the horst, includes major normal faults that strike east and northeast; the deepest part of the Norton basin (6.5 km) lies there. During the Late Jurassic and Early Cretaceous, basement rocks now beneath the Norton basin were affected by the orogeny that formed the Brooks Range. These basement rocks were metamorphosed and thrust, and then eroded deeply nearly 50 m.y. before the basin began to form. In the middle Late Cretaceous, the area of the eastern Seward Peninsula and eastern Norton Sound was subjected to east-west compression and consequent eastward thrusting. During the latest Cretaceous and early Paleogene, the compression gave way to regional extension that formed northeast-trending grabens onshore, east of Norton basin. I believe that the Norton basin most likely formed in response to this regional extension. Initial deepening of the basin was controlled by major normal faults and occurred rapidly, whe eas later subsidence was slower and more regional in scale. End_of_Article - Last_Page 569------------

Geology of Norton Basin and Continental Shelf Beneath Northwestern Bering Sea, Alaska

The rocks that floor the Norton basin and the northwestern Bering Sea are most likely of Precambrian and Paleozoic age, like those rocks that crop out around the basin. A maximum of 6.5 km of mainly Cenozoic strata lie over basement in the basin. On the basis of the geometry of reflections in seismic data, we believe alluvial fans to be present deep in the basin and to border major basement fault blocks. These fans are the lowest units of the basin fill in many areas and consist of uppermost Cretaceous or lower Paleogene, possibly coal- and volcanic-rich rocks. Mainly clastic nonmarine sedimentary rocks overlie the fan deposits. The Neogene and Quaternary basin rocks apparently were deposited in a marine environment. The basement of the northwestern Bering Sea, west of Norton basin, is 1 km below sea level for the most part, but local structural lows are filled with at most 2 km of strata of probable late Neogene and Quaternary age. The continental shelf in this area is separated structurally from the Seward Peninsula by a zone of small basement antiforms, over which relatively high gravity values have been measured. The Norton basin comprises two structural areas that are separated by a major northwest-striking horst. The first structural area lies west of this horst, where major normal faults strike northwest to form local areas where the basin is as deep as 5 km. The second area, east of the horst, includes major normal faults that strike east and northeast. The deepest part of the Norton basin (6.5 km) lies there. During the Late Jurassic and Early Cretaceous, basement rocks now beneath the Norton basin were affected by the orogeny that formed the Brooks Range. These basement rocks were metamorphosed and thrust, and then eroded deeply nearly 50 m.y. before the basin began to form. In the middle Late Cretaceous, the area of the eastern Seward Peninsula and eastern Norton Sound was subjected to east-west compression and consequent eastward thrusting. The strike-slip Kaltag fault probably formed at this time. If Norton basin formed as a pull-apart basin along this fault, then the basin may be as old as middle Late Cretaceous. However, during the latest Cretaceous and early Paleogene, the compression gave way to regional extension that formed northeast-trending grabens onshore, east of Norton basin We believe that the extensional Norton basin most likely formed in response to this regional extension. Initial deepening of the basin was controlled by major normal faults and occurred rapidly, whereas subsidence was slower and more regional in scale.

A Three-Dimensional Seismic Study of Challenger Knoll: ABSTRACT

Several intersecting multichannel seismic lines shot by The University of Texas Marine Science Institute in the deep Gulf of Mexico provide a three-dimensional grid in the vicinity of Challenger Knoll. Seismic stratigraphic sequences identified on the lines include the Sigsbee, Cinco de Mayo, Upper and Lower Mexican Ridges, Campeche, and Challenger units. Isotime End_Page 548------------------------------ maps document a detailed growth history of the Challenger diapir. DSDP drill holes 2, 3, and 87 provide age data for the grid. Differences in the style of diapirism between the northern (Louann) salt province and the Sigsbee salt province to the south are due to the influence of the basement. Both the original distribution of evaporites and subsequent diapirism in the vicinity of Challenger Knoll are controlled by basement fault blocks of transitional crust that formed during the initial opening of the Gulf of Mexico. End_of_Article - Last_Page 549------------

THE EYRE SUB-BASIN: RECENT EXPLORATION RESULTS

The Eyre Sub-basin of the Great Australian Bight Basin comprises a series of half-grabens with a maximum sediment thickness in the order of 6 000 m. It is bounded to the north by high-standing basement with a sedimentary cover about 550 m thick. To the west, sedimentary cover gradually thins and onlaps rising basement. To the south, a high- standing basement ridge separates the sediments within the Eyre Sub-basin from those of the Great Australian Bight Basin proper. The sedimentary pile apparently thickens south of the basement ridge where water depth increases to more than 1 400 m.The high basement trend bounding the sub-basin to the south plunges gradually to the east where it is eventually broken up by faulting. Seismic data from the eastern end of the sub-basin show progressive down-faulting of basement and increasing sediment thickness to the south.Jerboa 1 was drilled on a tilted basement fault block. It penetrated 1 739 m of sedimentary section, which is believed to be a condensed sequence representative of most of the total sedimentary fill of the sub-basin. Middle to Late Jurassic (Callovian-Kimmeridgian) sediments were encountered above basement, and the sequence continued almost unbroken into the Late Cretaceous (Cenomanian). Minor unconformities occur between the non-marine Aptian sequence and the overlying marine Albian, and between the Albian and Cenomanian. A major unconformity separates the Cenomanian from the overlying Tertiary section, interpreted to have been deposited after the separation of Australia from Antarctica.

Late Jurassic Reefs of Smackover Formation--Preliminary Report: ABSTRACT

Algal and coral reefs are recognized in conventional cores of the upper Smackover Formation from southwestern Arkansas eastward into the panhandle of Florida. Although only one known reef has produced commercial hydrocarbons, attractive porosities and permeabilities (mean ^phgr of 15%, mean K of 20 md) result from freshwater leaching, fracturing, or dolomitization. In addition, the reefs may have provided a positive structural aspect to localized areas during later Smackover deposition and diagenesis. Smackover reefs formed in the Late Jurassic during periods of maximum marine transgression (good circulation, clear water, normal marine salinity) in three major paleogeographic settings: (1) the margins of Paleozoic highs protruding into the Smackover basin, i.e., Vocation field in Alabama; (2) upthrown basement fault blocks, i.e., Melvin field in Alabama; and (3) the seaward edges of upthrown salt-cored fault blocks, i.e., Walker Creek field in Arkansas, Hico Knowles and North Haynesville fields in Louisiana, and West Paulding field in Mississippi. The buildups are commonly elongate, 3 to 40 m thick and generally cover an area of several square kilometers. The reefs appear higher (younger) in the stratigraphic section downdip. Also, the reefs are younger and have a more diverse biot in Arkansas and Louisiana than they do in Alabama and Florida. Smackover reefs in Alabama and Florida were constructed by algae. Vertical relief on the reef surface during growth may have been a few meters. Similar reefs in southern Arkansas and northern Louisiana exhibit a vertical zonation suggesting an evolving reef community. These buildups are Tubiphytes-stromatolitic algal boundstones containing scattered corals toward the base; diversity increases upward with the addition of abundant corals (Actinastrea, Complexastrea, Thamnasteria, and others), sponges, skeletal algae, and byrozoans. The reefs are commonly underlain and overlain by subtidal peloidal lime packstones containing oncolites and scattered fossils, and they can develop in close proximity to subtidal quartz sands. End_of_Article - Last_Page 1675------------

Results of DSDP Leg 77 in Deep Southeastern Gulf of Mexico--How Good Was our Seismic Stratigraphic Interpretation Ahead of the Drill?: ABSTRACT

A detailed seismic stratigraphic interpretation of an extensive grid of multifold seismic reflection data provides the basis for a model of the pre-middle Cretaceous sedimentary history of the southeastern Gulf of Mexico. The study area is located in the deep-water part of the western Straits of Florida between the Campeche and Florida Banks north of Cuba. The model predicts that: (1) block-faulted basement topography represents the top of a rifted and attenuated continental (transitional) crust; (2) a syn-rift sedimentary sequence possibly consisting of volcanic and nonmarine rocks of Triassic to Middle Jurassic fills in rift basins and covers the rift topography; (3) a post-rift sedimentary sequence representing a transition upward from shallow- to deep-marine rocks of ate Jurassic End_Page 907------------------------------ to middle Cretaceous (150 to 97 m.y.) was deposited on transitional crust and syn-rift strata as the crust cooled and subsided. This model was tested by four DSDP holes drilled on Leg 77 of the RV Glomar Challenger during December-January, 1980-81. Two holes were drilled to sample the thick pre-middle Cretaceous sedimentary sequences, and two holes were drilled to sample basement. Results of this drilling are presented and are compared with the model predicted by the seismic stratigraphic analysis. Data concerning the geologic history of this area, developed from the drilling and the seismic stratigraphic analysis, have important implications for future hydrocarbon exploration in adjacent shallow-water provinces such as the South Florida Bank, the Campeche Bank, and Cuba. End_of_Article - Last_Page 908------------

Geologic Setting and Oil and Gas Potential of Eastern United States Continental Margin North of Cape Hatteras: ABSTRACT

Two sedimentary basins, the Georges Bank basin off New England and the Baltimore Canyon Trough off the Middle Atlantic states, have been examined for geologic setting and hydrocarbon potential. Georges Bank basin, a complex-shaped trough, is on a block-faulted basement of igneous, metamorphic, and sedimentary rocks. The deepest part of the basin (deeper than 8 km) and the oldest sediments are restricted to south-central Georges Bank. Toward the northeast and southwest, the sedimentary section thins to less than 2 km over the Yarmouth Arch-LeHave platform and the Long Island platform. The only deep wells in the area are the COST G-1 and G-2; data from these wells will not be released until 60 days after the first oil and gas lease sale. Seismic correlation with the Shell M hawk B-93 well on the Scotian Shelf indicates that most of the sedimentary rocks in the Georges Bank basin are Jurassic and older. Jurassic sandstone and limestone units serve as potential reservoir rocks. Potential hydrocarbon traps may occur on structural highs associated with draping of Jurassic and basal Lower Cretaceous strata over basement blocks. The Baltimore Canyon Trough is an elongated northeast-trending basin that contains at least 14 km of Jurassic and younger marine and nonmarine sedimentary rocks. Lithologic and stratigraphic data from the COST B-2 and B-3 wells indicate that Lower Cretaceous and Jurassic rocks are predominantly nonmarine to shallow-marine sandstone and shale. Analyses of organic carbon and identification of low thermal maturity suggest that gas rather than oil will be produced. Nineteen wildcat wells have been completed; three are significant natural-gas discoveries. The largest discovery is probably associated with a large rollover trap on the downthrown block of a Cretaceous and Jurassic growth fault. Potential hydrocarbon traps in carbonate rocks beneath the present continental slope have not yet b en explored. End_of_Article - Last_Page 746------------

The geosynclinal pair at the continental margin of Peru

The Andean geosynclinal pair in Peru consists of a eugeosynclinal marine andesitic volcanic trough and a miogeosynclinal sedimentary trough. Both troughs developed on a block-faulted basement of Precambrian crystalline rocks in which movement occurred along faults parallel to the continental margin. Subsidence in the eugeosyncline was most rapid during the Albian during which period about 7000 m of marine volcanics were deposited. In the miogeosyncline the greatest subsidence took place during the Tithonian but continued throughout the Cretaceous to accumulate a total thickness of about 6000 m. From the Late Cretaceous to the mid-Tertiary the Andean granitoid batholith was emplaced, mainly in the eugeosynclinal zone. A Benioff zone has been active at the continental margin from the Early Jurassic until the present so that the entire geotectonic cycle from the geosynclinal stage through to orogeny and uplift has taken place under a regime of active subduction. During the geosynclinal stage the sedimentary troughs developed in fault-bounded blocks which subsided under tension. It is possible to envisage crustal thinning within the subsident blocks by rotational movement on extensional faults or by plastic stretching of the lower crust. Stretching would be facilitated by the high geothermal gradient resulting from the emplacement of plutonics and volcanics and might be analogous to the formtion of marginal basins of western Pacific type. It is more difficult to account for the subsequent uplift for although the granites contributed to the crustal thickening, the main uplift did not occur in the granitic sector but further inland, and was moreover delayed for at least 20 m.y. after the last granites were emplaced.

Reversal of Basement-Block Motions in Cambay Basin, India, and its Importance in Petroleum Exploration

The study of the nature of the basement and the reversals of the sense of movements along basement fault blocks are of great significance to petroleum exploration in the Cambay graben basin of western India. Oil accumulation in the basin is controlled primarily by basement relief and fault-block tectonics. The Cambay basin is a composite basin characterized by a two-stage structure. The Precambrian basement of the basin, composed of igneous and metamorphic rocks, is an extension of the Aravalli-Delhi, Satpura, and Dharwar orogenic belts trending, respectively, northeast-southwest, east-northeast-west-southwest, and north-south. The present basin apparently first began to form during the Mesozoic (Jurassic) when the region was an undisturbed shelf bounded on the east by the Indian shield. An intracratonic graben came into existence on this shelf during or immediately following the formation of the latest Cretaceous Deccan traps. A thick sequence of Tertiary sediments was deposited directly above the trap. During Paleocene-early Eocene time, the Olpad Formation was deposited on a Deccan t ap floor characterized by narrow fault blocks--horsts and grabens. Some of these fault blocks reversed their structural relations during the deposition of the early Eocene Cambay Shale. Such a reversal is exemplified by (1) the north-south Sanand-Nawagam-Dholka block, parallel with the Dharwar strike and (2) the east-northeast-striking Kosamba-Olpad block parallel with the Satpura strike. During the subsequent, post-Cambay Shale evolution of the basin, drape-fold structures were formed over fault scarps and/or horsts. Early Tertiary volcanism is conspicuous along the west coast of India and is manifested by flood basalts, dike swarms, and eruptive centers. The volcanic activity and the incipient faulting which led to the subsidence of the Cambay graben basin probably are related to the opening of the northwestern Indian Ocean along the Central Indian and Carlsberg Ridges. As a result of the subsidence of the basin floor, compressional stresses (stressing) were developed in the crustal layers. These stresses were relieved by the uplift of many fault blocks in the sagging basin floor. The location of the Cambay basin at the junction of three ancient Precambrian tectonic trends--the Aravalli-Delhi, Satpura, and Dharwar--is significant. The reversal of the fault blocks is believed to have been caused by unstressing. This stage may have been controlled by the rejuvenation and interaction of the three ancient tectonic trends. The unstressed blocks then were readjusted in response to forces such as gravity, isostasy, loading, and unloading. The history of fault-block movement and rejuvenation of the old Precambrian lineation can be applied directly to petroleum exploration. The relative position of a fault block in the basin controls the deposition of source materials and reservoirs within it. Furthermore, the time of reversal of basement blocks is of significance, both from the standpoint of dating the deposition of lenticular reservoir sandstones, and the development of unconformity traps.

Geologic Framework of Continental Shelf off Northwest Australia

Oil exploration on the continental shelf off northwest Australia has been significantly active since oil was discovered at Barrow Island in 1964. The search has accelerated so that sufficient data have been accumulated to allow a reasonable gross analysis of the shelf's geologic framework. Reconnaissance aeromagnetic, gravity, and seismic surveys have provided evidence of substantial thicknesses of sedimentary rocks and revealed the presence of several major structural-depositional features. The geologic significance of these features has been partly rationalized by the integration of well and seismic data, and by interpolation of established onshore geology. Two offshore wells, isolated from geologic control, have provided vital stratigraphic information. Ashmore Reef No. 1 revealed a substantially complete Tertiary-Upper Cretaceous carbonate-clay sequence, a thin Lower Cretaceous-Upper Jurassic section containing detritus from underlying Upper Jurassic basic lavas, and part of a thick Triassic sedimentary sequence. Legendre No. 1 penetrated Tertiary, Cretaceous, and J rassic sedimentary sequences with gross stratigraphic similarities to sequences in parts of the Carnarvon basin; oil was discovered in Lower Cretaceous sandstone. The shelf has a block-faulted basement substructure of Precambrian rocks related in part to the extension of major structure in Precambrian rocks of the northwest Australian mainland. The geologic superstructure of the shelf consists of warped and faulted Phanerozoic sequences which have two major strike components, one subnormal and the other subparallel with the present coastline. The subnormal trend is considered to be related to rejuvenated Precambrian structure and the subparallel trend was superimposed subsequently by later Phanerozoic tectonic events of predominantly noncompressional origin.

GEOLOGICAL FRAMEWORK OF THE CONTINENTAL SHELF OFF NORTHWEST AUSTRALIA

A simplified synthesis of exploration results in permits covering a large area of the broad continental shelf off northwest Australia (between Bathurst Island, N.T. and the Monte Bello Islands. W.A.) is presented and a preliminary analysis of the geological framework of the shelf is offered. Companies with working interests in these permits are B.O.C. of Australia Limited. Shell Development (Australia) Pty. Ltd., Woodside (Lakes Entrance) Oil Co. N.L., Mid-Eastern Oil N.L., B.P. Petroleum Development Australia Pty. Ltd. and. in the permits In Western Australia, California Asiatic Oil Co.Reconnaissance aeromagnetic and seismic surveys have provided evidence of substantial thicknesses of sedimentary rocks, and revealed the presence of several major structural/ depositional features. The geological significance of these features has been partly rationalized by the integration of stratigraphic and seismic data, and by interpolation of established onshore geology. Ashmore Reef No. 1 revealed a substantially complete Tertiary-Upper Cretaceous carbonate- clay sequence, a thin early Cretaceous-late Jurassic section, containing detritus from underlying Upper Jurassic basic lavas, and part of a thick Triassic sedimentary sequence. Legendre No. 1 penetrated Tertiary, Cretaceous, and Jurassic sedimentary sequences, with gross stratigraphic similarities to sequences in parts of the Carnarvon basin.The shelf has a block-faulted basement substructure of Precambrian rocks, related in part to the extension of major structure in Precambrian rocks of the northwest Australian mainland. The geological superstructure of the shelf consists of Phanerozoic rocks whose essential structural framework is related to rejuvenated Precambrian structure. Warped and faulted Phanerozoic sequences have two major strike components, one sub-parallel and the other sub-normal to the present coastline. The effects of tectonics on sedimentation are outlined.

Geological History of Central and South-Central Montana

Central Montana has had a complex structural and sedimentary history, especially in the area of the present-day Central Montana uplift. Precambrian and Cambrian subsidence was accompanied by the deposition of more than 13,000 feet of clastics in an east-west-trending trough roughly coincident with the present-day uplift. Pre-Devonian uplift and erosion took place after stable depositional conditions during Ordovician time. Ordovician strata have been eroded from the western half of the study area. Silurian beds are absent from the entire study area. The Central Montana uplift area remained high during Early, Middle, and part of Late Devonian time. Upper Devonian rocks lap onto the uplift, and uppermost Devonian sediments finally covered the area. Pre-Mississippian uplift and erosion removed latest Devonian carbonates and shales completely from a large area of the uplift. The Mississippian System is composed of the carbonate-evaporite Madison Group and clastic Big Snowy Group. Stable conditions prevailed through most of Madison deposition, but central Montana began to subside in late Madison time. Distribution of the Big Snowy Group was restricted by continued subsidence which downwarped central Montana into a synclinorium. Early Pennsylvanian streams draining eastern Montana cut valleys in the Central Montana trough. These valleys were filled, as the streams attained old age, primarily with sandstones and shales derived from the underlying Big Snowy Group. These stream-channel deposits, comprising the lower member of the Tyler Formation, contain the major reservoirs of central Montana. Middle and Upper Pennsylvanian sediments covered central Montana, but all of the Upper Pennsylvanian was eroded before Jurassic time. Pre-Jurassic folding accentuated Mississippian structure. During Jurassic time uplift occurred in the Belt Mountain area at the west. Jurassic strata lap onto this high and thicken regionally eastward and in the Central Montana trough area. Lower Cretaceous deposition was controlled by uplift at the south. These sediments thicken from south to north. The Laramide orogeny upwarped the old trough to form the Central Montana uplift and generally folded the old synclines into anticlines as, for example, at the Sumatra trend. Differential epeirogenic movements, which probably resulted from isostatic adjustment of basement fault blocks, caused the down-up-down-up movements of central Montana. The wedging and truncation of Ordovician, Devonian, Mississippian, and Pennsylvanian reservoir beds created numerous prospects for oil and gas in and around central Montana. Zones of stratigraphic changes in the Pennsylvanian, Jurassic, and basal Cretaceous are also prospective. The area should be rewarding to the explorationist.