Cambrian-Ordovician Rocks Research Articles

Lenticular Stretch Structures in Eastern Nevada--Possible Trapping Mechanism in Supposed Graben: ABSTRACT

Eastern Nevada is widely recognized as a region of tectonic extension. The dominant structures are generally agreed to be low-dipping, younger over older faults and steeper listric faults that are responsible for the basins (grabens) and ranges (horsts). In the Schell Creek-Duck Creek Range, east of Ely, and in the White Pine Range, southwest of Ely, small lenticular structures bounded by tectonic discontinuities can be clearly seen in the field. These lenticular units, or stretch structures, range in length from a few meters to more than 200 m. All lenticular stretch structures that can be clearly seen in the field are stratigraphically restricted; the stretched formations are the Eureka Quartzite, the Pilot Shale, the Joana Limestone, and the Chainman Shale. Still larger stretch structures, which may include several formations, are inferred, and the authors suggest that extension has created lenticular structures at all scales. The Duck Creek and Schell Creek Ranges east of Ely consist mostly of Devonian and older rocks. They are separated by a topographically lower area containing mostly Mississippian and Pennsylvanian rocks. This structure, which separates the ranges, has been referred to as a graben, but field evidence suggests that it is a large-scale lenticular stretch structure.more » Unlike a true graben, the structure does not extend downward. For example, in several places within the supposed graben, Cambrian and Ordovician rocks project through a cover of Carboniferous Chainman Shale and Ely Limestone, suggesting the Chainman-Ely is a thin sheet underlain by Cambrian-Ordovician rocks. Accordingly, they suggest that extension in the Duck Creek-Schell Creek Ranges stretched the formations into lenticular bodies. Between the Duck Creek and Schell Creek Ranges, the Cambrian-Ordovician is attenuated, and the resulting tectonic depression is occupied by a lenticular mass of Carboniferous rocks.« less

Structural Complexity Associated with Transfer Zone: An Example from Southern Appalachians: ABSTRACT

Transfer zones provide a mechanism by which displacement is maintained across a fold and thrust belt where a single thrust terminates along strike. Commonly, the transfer is not simply a lap joint wherein en echelon thrusts, tied to a common sole fault, shift displacement; rather, these zones are structurally complex with intensified folding, fracturing, and, in some instances, cleavage. An example occurs in the Valley and Ridge of eastern Tennessee, northeast of Knoxville. There, the Beaver Valley thrust terminates into folded Middle Ordovician carbonates. To the southeast, the adjacent Saltville thrust increases in stratigraphic separation, coinciding with the Beaver Valley termination. The Saltville places Lower Cambrian strata on Mississippian units northeast of the fault termination, whereas the Lower Cambrian rests on Cambrian-Ordovician rocks only 20 km to the southwest. Near its tip, the Beaver Valley thrust shows little displacement, but the loss of slip is accompanied by intensified folding. The folding traces an anticlinorium involving Middle and Upper Ordovician carbonates and shales. These folded rocks are broken by fractures and extensional veins, commonly filled with sparry twinned calcite, and ranging from 0.2 to 2.0 mm wide. One set of veins is generally normal to bedding with a second set oblique; in some cases, three sets cut the rock. Also, Middle Ordovician carbonates carry an axial planar solution cleavage, and catacalcites are locally developed, particularly in dolostones of the Cambrian-Ordovician Knox Group. A set of folds, the axes of which are oblique to transport direction, occurs across the transfer zone; they are often found in he footwall of a thrust. The complex structural geometries in these thrust sheets suggest contradictory timing relationships. The contradiction is explained if the two faults are envisioned as propagating from initial breaks, then growing toward one another contemporaneously. This example demonstrates the potential complexity of a transfer zone, as well as the possibility of apparent local variation in fault sequence in a generally hinterland to foreland progression of emplacement. End_of_Article - Last_Page 1440------------

COCORP profiling across the Southern Oklahoma aulacogen: Overthrusting of the Wichita Mountains and compression within the Anadarko Basin

COCORP (Consortium for Continental Reflection Profiling) deep reflection profiles recorded across the Wichita Mountains and Anadarko Basin suggest that significant crustal shortening occurred in the final stages of the evolution of the Southern Oklahoma aulacogen. The crystalline rocks of the Wichita Mountains were thrust in Pennsylvanian time northeastward over sedimentary rocks of the Anadarko Basin along a series of faults with moderate (average 30° to 40°) and southwesterly dips. These faults can be traced possibly as deep as 20 to 24 km. Listric thrust faults and hanging-wall anticlines developed in the sedimentary rocks of the basin. These features contrast with conventional interpretations of Pennsylvanian structures as the result of predominantly vertical movements along high-angle faults, and they suggest that Pennsylvanian downwarping of the Anadarko Basin was at least partially due to thrust loading. Truncations of reflections from Cambrian-Ordovician rocks in the deepest part of the basin suggest normal faulting, which would support ideas of an early extensional stage in the aulacogen cycle. The distinctive Precambrian layering seen on earlier COCORP data recorded south of the Wichita Mountains cannot be recognized under the Anadarko Basin, and the Proterozoic basin containing that layering may have been bounded on its north side by a Precambrian fault. This inferred fault was probably twice reactivated during formation of the Southern Oklahoma aulacogen—once during late Precambrian(?)-Early Cambrian extension, and again during Pennsylvanian compression. The popular view that aulacogens originated from radial rifting of updomed, homogeneous continental crust is probably too simplified, and a more important constraint on their location and development may be the nature of pre-existing lines of weakness.

Ravia Nappe, Bryan County, Oklahoma: A Gravity Slide Block off the Tishomingo Uplift: ABSTRACT

The Ravia nappe in Bryan County, Oklahoma, is located along the southwestern flank of the Tishomingo uplift, between the Cumberland and East Durant oil fields. This mass of Cambrian-Ordovician through Mississippian sediments tectonically overlies younger Springer shales (Pennsylvanian) of the Ardmore basin. Previously, this feature has been interpreted to have been thrust southward along the Cumberland fault, a fault parallel to the Ravia thrust. Reinterpretation of this area, with additional well data, indicates the Ravia nappe is a gravity slide block off the uplifted Tishomingo mountains. The Ravia nappe subcrops below the Cretaceous unconformity as a 16 mi2 (41 km2) triangular-shaped mass with a maximum thickness of 4,800 ft (1,450 m). The fault surface along its base has a bowl-like shape. This surface is neither the Cumberland fault nor the Ravia fault, but appears to be a separate fault surface. The nappe consists of overturned Caney through Simpson rocks (Mississippian-Ordovician) along its southwestern edge (toe), whereas right-side-up Arbuckle carbonates (Cambrian-Ordovician) occur on the northeastern side (heel) of the nappe. Arbuckle carbonates of the nappe overlie an overturned-to-the-south syncline of Arbuckle through Springer rocks. This overturned syncline, on the footwall side of the Ravia fault, is present north of the Cumberland oi field and trends southeastward into Sec. 9, T6S, R8E. At this location the structural style of the Tishomingo footwall rocks changes from an overturned syncline south of the Ravia thrust to another thrust with a footwall fold. The Ravia thrust possibly terminates near this change in structural style with the other northeast-dipping thrust continuing southeastward to the East Durant oil field. The Ravia nappe is interpreted to have been originally the southwest overturned limb of the Tishomingo uplift. Prior to the major thrusting on the Ravia thrust, but after compressional folding and uplift of the Tishomingo mountains, a breakaway fault formed across the most intensely folded beds. This breakaway fault undercut the overturned southwestern limb of the Tishomingo uplift in a concave-upward fault surface. Gravitational forces caused the Ravia nappe Mississippian Caney rocks to Cambrian-Ordovician Arbuckle rocks to slide rotationally southwestward 2.5 mi (4 km). Topographic relief prior to the slide may have been as much as 9,000 ft (2,700 m). The slide occurred sometime during late Morrowan to early Desmoinesian. Analogs of this type of deformation are present in the Owl Creek Mountains, central Wyoming; Front Range, northern Colorado; Qal'eh Raisi, southwestern Iran; and Belton anticline and Sulphur syncline, Oklahoma. These analogous structures and the Ravia nappe, show three common elements: (1) they are competent, erosion-resistant units which slide off the flanks of folded anticlines across softer shaly units; (2) the heel portion of the slide surface is a bedding plane fault, whereas the toe of the slide surface cuts across bedding planes; and (3) the driving mechanism is the gravitational force associated with tectonic uplift. End_of_Article - Last_Page 1325------------

Analysis of Simultaneously Acquired Airborne Gravity and Aeromagnetic Data in Eastern Overthrust Area: ABSTRACT

Continuously recorded airborne gravity surveying is a new airborne geophysical technique, conducted in a gridded survey mode in combination with simultaneously acquired aeromagnetic data. The interpretation of the two data sets was achieved by fitting a series of layered structural surfaces to the gravity data, guided by the approximate depth to critical subsurface sources obtained from detailed analysis of the magnetic data. The interpretation of the aeromagnetic survey data indicates the presence of a very deep horizon in excess of 25,000 ft (1,620 m) in some areas. Although the possibility that these depths arise from intrabasement sources is strong, a structural surface based on the gravity field can be constructed to conform with this horizon which appears reasonable The more common depth to probable crystalline basement derived from the magnetic data is about 15,000 to 20,000 ft (4,570 to 6,100 m). The relief that occurs on this surface is again obtained from the analysis of gravity data. The importance of being able to correlate both surfaces adds considerable credibility to the interpretation. It is believed that the major thrust surfaces may be identified by the recognition of density contrasts between the Cambrian-Ordovician rocks, particularly the limestones, and the overlying shales. In addition, through the correlation of the magnetic and gravity data, the depth and the relief on the thrust sheets as well as on the basement surface can be determined. Wedges of crystalline rock from the basement apparently incorporated into the thrust sheet in some locations are also suggested by the magnetic data, and this is supported by small increases in the density contrasts. These implications of the subsurface have great significance in the exploration for petroleum in the area. End_of_Article - Last_Page 1708------------

The post-orogenic history of the Adirondack Mountain region: A review

A number of episodes in the post-orogenic history of the Adirondack Mountains and adjacent Paleozoic Lowlands are recognized. A late Hadrynian—early Cambrian tensional episode is suggested by the presence of nonmarine graben facies and extrusive volcanics. The Ottawa-Bonnechere graben was active at the initiation of Paleozoic deposition in the region. Subdued topography of the Adirondack massif had generally minor but locally important influence on early Paleozoic depositional patterns, as did minor basement faulting. Stratigraphic evidence suggests that the entire massif was covered by Cambrian-Ordovician cratonic sedimentary rocks. Following Cambrian-Ordovician sedimentation, a number of minor tectonic events can be recognized. These include renewed block faulting in the Ottawa-Bonnechere trend, normal faulting of pre-Silurian age on the southern Adirondack flank, post-Ordovician lead-zinc mineralization in Cambrian-Ordovician cover rocks, and intrusive activity of Jurassic-Cretaceous age. These later events may be related to either Taconian or Acadian orogenies. There is little evidence to suggest any major shortening of either basement or Paleozoic cover by post-Grenville tectonism in the region. Recent earthquake activity and Tertiary block faulting on the eastern Adirondack flank may be related to renewed uplift in the region.

Petroleum Developments in North Africa in 1979

In the 6 countries covered by this report, the extent of valid petroleum rights, seismic work, and drilling was nearly the same as in 1978. The success rate of wildcat drilling decreased slightly, to 28% (33% in 1978), with 26 oil or gas discoveries. In southwestern Tunisia, the Amoco Sabrina Nord 1 tested 930 bbl of 39° API oil from Cambrian-Ordovician sandstones--the first oil to come from lower Paleozoic rocks in Tunisia. First commercial oil from Cambrian-Ordovician rocks in western Libya was discovered by Agip A1-NC40 which flowed 1,400 b/d. Highlight of the year in North Africa was in the Interior basin of Sudan where the Chevron Abu Gabra 1 tested 900 BOPD of 40° API oil from Cretaceous rocks; 2 other wells, spudded in late 1979 in the same area, have tested 3,200 and 7,300 b/d, respectively, in early 1980. Discovery well of the Interior basin was Chevron Unity 1 which tested small amounts of oil in 1978. Oil production in North Africa in 1979 averaged 3,939,500 b/d compared with 3,802,800 b/d in 1978, an increase of 3.6%.

Reflectance Studies on Organic Matter in Lower Paleozoic Sediments of Quebec

ABSTRACT Reflectance data from twelve wells (six in St. Lawrence Lowlands Cambrian-Ordovician rocks, three in Silurian rocks, and three in Devonian rocks of Gaspe Peninsula) are included in this paper. The observations were made on polished organic matter obtained by palynological preparations made from drill cuttings and cores. The organic matter in the Cambrian-Ordovician and Silurian rocks, and the Devonian carbonates, presumably consists of asphaltic pyrobitumen, which closely resembles vitrinite under the microscope. The organic matter in the Devonian clastics is characteristically coaly. The relatively undeformed platform rocks of the St. Lawrence Lowlands, despite a wide regional variation, generally attain an initial maturity (oil prone) stage of organic metamorphism to a depth of approximately 3,000 ft (900 m) and a mature (mainly gas-with-condensate prone) stage to an approximate depth of 6,000 ft (1,800 m). These maturation levels are shown by the Utica and Lorraine shales, silts and minor sandstones, and the Trenton carbonates. Rocks below 6,000 ft (1,800 m) contain mainly gas possibilities. The Silurian sediments, in the Gaspe Peninsula area, generally attain mature and postmature stages of maturation corresponding to gas potential. The maturation can be correlated with the total thickness of Silurian rocks but anomalous high maturation levels in the interior of Gaspe Peninsula are attributed to regional igneous activity. The Devonian carbonates and clastics are best preserved in a series of regional folds in eastern Gaspe. They are at the immature to postmature stages of organic metamorphism, suggesting both oil and gas possibilities. The metamorphism increases toward the interior of Gaspe Peninsula, where it may be related to basinal thickening and increased regional igneous activity.

Growth Faults in Cambrian and Lower Ordovician Rocks of Western Pennsylvania

A Cambrian basin lies beneath the Appalachian Plateau of western Pennsylvania; it trends northeast-southwest between shelf sediments of northwestern Pennsylvania, where most of the Cambrian wells are drilled, and the overthrust outcrop belt of central Pennsylvania. The western edge of the basin is believed to be bounded by growth faults which restrict to the basin a thick sandstone herein named the Olin Sandstone. Evidence for growth faults is derived from many changes in regional stratigraphy of deep wells in Potter and Armstrong Counties, Pennsylvania, and Marshall County, West Virginia. The presence of growth faults also is postulated in Lower Ordovician rocks east of the faulting in the Cambrian strata. By projecting the Cambrian and Early Ordovician growth faults to he Precambrian structural contour map of Beck and Mattick, a Precambrian basin is indicated with structural relief of several thousand feet (about 1,200 m).

Rb-Sr geochronology and tectonic setting of the Peekskill pluton, southeastern New York

The Peekskill Granite pluton in southeast New York is a steep-walled, shallow intrusion in a major fault zone between Grenville rocks of the Reading Prong and Cambrian-Ordovician rocks of the Manhattan Prong. Emplacement of the pluton postdated folding during the Taconic orogeny and postdated the intrusion of the Cortlandt mafic complex. The pluton was probably emplaced during or following the development of the fault zone. Rb/Sr whole-rock analyses show that the pluton crystallized about 371 ± 14 m.y. ago with an initial 87 Sr/ 86 Sr ratio of 0.7074 ± 0.0002. The field relationships and age determinations indicate that the northwestern part of the Manhattan Prong was affected by brittle fracture and faulting and by igneous activity during the Acadian deformation.

Significance of Authigenic K-feldspar in Cambrian-Ordovician Carbonate Rocks of the Proto-atlantic Shelf In North America

ABSTRACT Strata containing abundant authigenic feldspar, which heretofore have been considered rarities in the geologic column, now appear to be characteristic of the Cambrian-Ordovician rocks that formed on the shelf of the proto-Atlantic Ocean. They crop out from the mid-continent of North America to the present Appalachian Mountain chain and along the paleo-shelf edge through Newfoundland, Scotland, and Greenland. Abundant authigenic K-feldspar deposits in Cenozoic alkaline lake beds of the western United States are known to have formed from tephra by replacement of an intermediate zeolite stage. The presence of some volcanic rocks of probable Cambrian and Early Ordovician age in New England suggested the possibility that the authigenic K-feldspar of the shelf rocks is the only remaining ev dence of older tephra. Chemical analyses of over 1,300 carbonate rock samples of Cambrian to Devonian age from the shelf in New York State indicate that Cambrian and Lower Ordovician strata contain 7 times more normative K-feldspar than the younger strata. Petrographic study utilizing cathodoluminescence of the Tribes Hill Formation of Early Ordovician age shows that the K-feldspar is not entirely authigenic but rather, is composed of subequal amounts of detrital K-feldspar as grain cores and authigenic K-feldspars as overgrowths. Detrital quartz always is associated with the K-feldspar. A portion of the Canadian Shield near the present New York area was exposed during the Cambrian and Early Ordovician and probably served as the source for the K-feldspar and quartz detritus deposited on he shelf during that time. Tephra may have provided potassium, aluminum and silicon ions to the formation of the abundance of K-feldspar overgrowths, although no direct evidence was found. By Middle Ordovician time that part of the shield which served as the source of detritus was submerged, and the supply of detrital K-feldspar necessary for the growth of authigenic K-feldspar ended.

Cambrian and Ordovician Oil Potential of Illinois Basin: ABSTRACT

More than half of the sedimentary rocks of the Illinois basin are of Cambrian and Ordovician ages. Within the area of the basin where younger Paleozoic rocks already have produced about 2.9 billion barrels of oil, these older sediments still remain virtually unexplored. Three holes have tested the Cambrian and Lower Ordovician rocks within the productive region. The Galena (Trenton) near the top of the Ordovician is productive on the western and northeastern flanks of the basin, but has been tested by less than one well per thousand square miles in the deep part of the basin and on the southern flank. Cambrian and Ordovician rocks probably are more than 6,000 feet thick in much of the deep part of the basin. They thicken and become finer-grained and darker basinward, indicating that a basin structure was present during early Paleozoic time. The rocks appear to be entirely marine; they contain brines whose salinity is more than 10 per cent. Dolomite, sandstone, shale, and limestone are present in that order of abundance. Indications of hydrocarbons in these beds on the basin flanks have been slight. Sparse seismic data in the basin and drilling in neighboring provinces indicate that structures beneath the pre-Middle Ordovician unconformity are complex and correspond only in part to those in the younger rocks. Drilling depths of 6,000-14,000 feet, which would be required to test these older rocks, are not great by modern standards. End_of_Article - Last_Page 636------------

GEOLOGY OF THE BUFFALO MOUNTAIN-CHEROKEE MOUNTAIN AREA, NORTHEASTERN TENNESSEE

The Buffalo Mountain-Cherokee Mountain area in northeastern Tennessee includes about 45 square miles and is located along the southeast border of the Appalachian Valley and Ridge geomorphic province. The mountainous part of the area is underlain almost entirely by the Buffalo Mountain thrust sheet, which has been separated by two minor thrust faults into three imbricate thrust blocks. Cambrian and Precambrian (?) rocks in the Buffalo Mountain thrust sheet consist of the Unicoi, Hampton, and Erwin formations (Chilhowee group) and the Shady dolomite. Younger, Cambrian-Ordovician rocks beneath the thrust sheet include the Honaker limestone, Nolichucky shale, Knox dolomite, and Athens shale. During or following the thrusting, all the rocks in the area were folded into a synclinorium trending northeast-southwest. Some folding apparently preceded the thrusting. Several “shear faults” mapped by Keith in this area do not appear to exist. An interesting feature of the structure is the number of slices that have been broken off and dragged along the thrust surfaces. Slices of younger rocks have been found between older rocks, and slices of older rocks between younger. Cleavage and a low-rank metamorphism are present. Deformation probably occurred in late Paleozoic time during the Appalachian orogeny.

Older Rocks of Van Horn Region, Texas

Some new facts are given, based on recent detailed surveys, regarding the pre-Cambrian, possible Cambrian, and early Ordovician rocks of the Van Horn region. A geosyncline existed here in later pre-Cambrian time, and in this a thick mass of sediments accumulated, constituting the Allamoore and Hazel formations. These were afterwards deformed, and were overridden from the south by a thrust block of an older formation, the Carrizo Mountain schist. After the deformed rocks were deeply eroded, the clastic, unfossiliferous Van Horn sandstone was laid down over them. Its age is unknown, but may be late pre-Cambrian or early Cambrian. It was tilted, faulted, and eroded before the first marine Paleozoic deposits were spread over it. These are of early Ordovician age, and correlat with the Bliss sandstone farther west. The type Bliss has hitherto been classed as Cambrian, but paleontological evidence for this is questionable. If the suggested correlation is correct, wide areas in western Texas and southern New Mexico, where the Bliss is the basal Paleozoic formation, may never have been covered by the Cambrian seas.