Paleozoic Sedimentary Section Research Articles

Mineralogical and geochemical studies of copper mineralization in the Paleozoic sedimentary section in southwestern Sinai, Egypt

The present study provides new insights into the distribution and characteristics of copper-bearing Paleozoic clastic-carbonate units, Araba and Um Bogma formations in southwestern Sinai, Egypt. Malachite, azurite, atacamite, chrysocolla, and cuprite occur as disseminations or stratiform, layer-parallel streaks, and lenticular bodies that are intercalated with vanadiferous sandstones in the middle unit of the Araba Formation. The vanadiferous sandstones are rich in mottramite, mimetite, duftite, beudantite, descloizite and vanadinite. The Um Bogma Formation is characterized by uncommon chalcanthite, antlerite, malachite, chrysocolla, covellite, and chalcocite in addition to traces of pyrite and barite. These ore minerals occur as acicular, fibrous, fracture-filling, disseminated, and pore-filling patches and are associated with Mn-minerals. Copper minerals form also thin laminae in the manganiferous and black organic-rich shales of the Um Bogma Formation.Copper and vanadium minerals are disseminated in the arkose, subarkose, sublitharenite, graywacke, shale, siltstone and dolostone members of the host formations. Elevated copper concentrations (up to 5.04 wt%) and vanadium (≤362.3 ppm) are associated with high CaO and MgO contents in the intergranular patches and discontinuous laminae. In the Um Bogma Formation, the mineralized layers show high contents of MnO (up to 84.27 wt%) and Fe2O3 (up to 72.1 wt%), Ba (0.03–2.86 wt%), S (up to 0.452 wt%) and Ni (0.012–0.041 wt%).Mineralogical and textural characteristics of the studied rocks indicate that copper and vanadium minerals in the Araba Formation are related to supergene processes and telodiagenesis after oxidation of sulfides and dissolution of feldspar grains and carbonate cement in the clastic rocks most probably during the Upper Devonian-Lower Carboniferous Hercynian event. In the Um Bogma Formation, copper mineralization is a supergene alteration of hydrothermal Fe-Mn minerals and associated primary Cu-Zn sulfides. Hydrothermal fluids likely mobilized several metals, i.e., Fe, Mn, and Cu, within dissolved and karstified carbonates in the Um Bogma Formation.

Paleozoic sequence stratigraphy, depositional systems, and hydrocarbon habitats across the Arabian plate

The Paleozoic sedimentary section of the Arabian plate recorded the effects of numerous fluctuations in sea level and gentle epeirogenic movements. Eustatic changes in sea level over the Arabian plate resulted in transgressions and regressions and created many local and regional unconformity surfaces, identified by missing section and nondeposition. The distant deformation of the Caledonian tectonic orogeny and mountain building of the North Atlantic region, and the later Hercynian tectonic event, are partially responsible for eustatic sea-level changes. Additionally, detailed evidence of Paleozoic glacial erosion and glacial eustasy in the regressive sedimentary section over uplifted paleohighs comes from deep wells penetrating local exotic tectonic blocks. The resulting surfaces mark the sea-level falls associated with the glacial events in the Ordovician, Late Devonian, and Carboniferous–early Permian sediments. The depositional settings of the Cambrian–upper Permian rocks include deltaic and glacial settings and are expressed as alluvial–fluvial fans that grade into braided plains and shallow-marine carbonates flanked by distal shales. Late Permian sediment fill accompanied the development of widespread accommodation, resulting from a major sea-level rise, coinciding with a major period of carbonate–evaporite deposition, in which clastic sediments were only a minor factor. The regional sedimentary facies have good reservoir quality and are juxtaposed to regional source rocks and seals associated with both structural and minor stratigraphic traps. Production occurs in Precambrian through Permian strata in which mature clastic and shallow-water carbonate reservoirs occur, commonly sealed by shales, and sourced by Silurian hot shales or older source rocks in the section.

Caledonian detachment deformation and deposition in the Fuling gas field of the southeastern Sichuan Basin in China: Implications for the Lower Silurian Longmaxi Shale gas exploration and production

The Fuling gas field in the southeastern Sichuan Basin is the first and the largest shale gas play in China that has been producing primarily from the organic-rich shale in the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation. Newly processed 3D seismic data along with well-completion data in the gas field reveal important structural, depositional, and reservoir details in the Lower Paleozoic sedimentary section. Lateral (along-stratal) variations in time structure and bed curvature demonstrate the diversity in faults that can be classified based on their orientation (regional and cross-regional), scale (small, intermediate, and large), and mode (contractional, extensional, and wrench). Vertical (cross-stratal) variations in time structure and bed curvature demonstrate that the deformational intensity increases from the Lower Cambrian to the Upper Ordovician, then decreases from the Upper Ordovician to the Silurian. Seismic isochron and facies analyses indicate that the structural deformation influenced the syntectonic deposition of turbidite sand in a channel complex above the reservoir. The pore pressure, porosity, and gas productivity of the reservoir are the highest in the central portion of the field, where small-scale faults dominate, but drop significantly at the edges of the field, where large-scale lineaments dominate. The relationships suggest that faults and fractures could either reduce or enhance pore pressure, porosity, and gas productivity, depending on their scale. Large-scale faults have the most negative impact on gas enrichment and pressure build-up, leading to reduced pressure, porosity, and productivity; whereas, small-scale ones have the least negative or even positive impact on gas enrichment and pressure build-up, leading to increased pressure, porosity, and productivity. These observations and interpretations offer new insight into the dynamic interplay among tectonic deformation, syn-tectonic sedimentation, and reservoir integrity during the Caledonian (Late Ordovician to Silurian) in the southeastern Sichuan Basin (China).

Offshore extent of Gondwanan Paleozoic strata in the southeastern United States: The Suwannee suture zone revisited

The recognition of the Suwannee terrane in southeastern North America was a critical step in advancing our current understanding of the tectonic evolution of ocean basins. Unfortunately, further clarification of the boundaries of the Suwannee terrane has proven difficult due to the thick coastal plain cover, limited well log analyses, and a paucity of seismic reflection data onshore. We present the results from a new compilation and analysis of legacy marine seismic reflection, refraction and well data from offshore the southeastern United States, which reveals in the subsurface a previously unmapped lower Paleozoic sedimentary section spanning the continental shelf from Florida to North Carolina. Previously only identified in two deep offshore wells (COST GE-1 and Transco 1005-1), this Paleozoic platform sequence is identified in seismic reflection profiles as a package of low-frequency, sub-horizontal, laterally-continuous reflectors which are clearly discordant with and distinct from the overlying Mesozoic and Cenozoic stratigraphy above the post-rift unconformity (PRU). The inferred base of the Paleozoic sequence is marked by the downward diminution of parallel seismic reflectors, as well as an increase in seismic velocities to >6km/s observed on numerous offshore seismic refraction surveys. While clearly deformed in some areas, these Paleozoic strata are generally sub-horizontal, range in thickness from 4 to 6km, and can be mapped continuously over an area in excess of 130,000km2. Similar sedimentary rocks have been recognized from onshore exploration wells in Florida since the 1930s, and were subsequently identified to be part of the Suwannee basin within the larger exotic Suwannee terrane of Gondwanan affinity. Recognition of the presence and extent of these Gondwanan strata offshore implies: (1) the inferred position of the Suwannee suture zone offshore lies >200km further north, approximately along the boundary between the Carolina terrane and the Charleston terrane; (2) previously identified terranes (Brunswick, Charleston, Suwannee, Northern Florida) are likely distinct crustal blocks, but their amalgamation predates deposition of the overlying Paleozoic section of the Suwannee basin; (3) collectively, this crust represents the Gondwanan continent based on the size and presence of a stable platform stratigraphy, nominally doubling the size of the last sutured terrane, the Suwannee terrane.

Reservoir quality and K-Ar age dating of the pre-Khuff section of Kuwait

ABSTRACT Based on studies of petrographic thin sections from core and cutting samples, the pre-Permian siliciclastics in four deep wells in southern Kuwait were found to be tight. Three of these wells are located on the crestal region of the Burgan Arch, and one on the Umm Gudair anticline. These clastics were encountered beneath a thin brick-red shale of unknown thickness, immediately below the pre-Khuff unconformity at the base of the Permian-Triassic Khuff Formation. The pre-Khuff clastics range in thickness from a few tens of feet to more than 4,000 ft, and overlie a Proterozoic argillite (Economic Basement). Based on Illite Age Analysis (IAA) of samples from cores, the depositional K-Ar age of the pre-Khuff clastics is estimated to be younger than 509 Ma (90 percent confidence interval: 544–481 Ma, i.e. Cambrian-Early Ordovician). The argillite was uplifted through the 300°C isotherm at about 611 Ma (90% confidence interval: 635-588 Ma, i.e Late Proterozoic); its deposition and metamorphism preceded this date. During the Paleozoic, the pre-Khuff clastics were buried to depths of 10,000–15,000 ft, but were subsequently uplifted in the Late Paleozoic. IAA diagenetic K-Ar ages of the Economic Basement (421 Ma; 90 percent confidence interval: 442-397 Ma; Late Ordovician-Early Devonian) and pre-Khuff clastics (369 Ma; 90 percent confidence interval: 404–337 Ma, i.e. Devonian-Early Carboniferous) indicate that by these times the pre-Khuff section was already deposited and undergoing burial diagenesis. The interpretation of gravity data indicates that in Paleozoic basinal regions (e.g. between the Burgan Arch and Umm Gudair Anticline), the Paleozoic sedimentary section is likely to be more complete and may exceed 10,000 ft in thickness.

Interpreting the Earthquake Source of the Wabash Valley Seismic Zone (Illinois, Indiana, and Kentucky) from Seismic-reflection, Gravity, and Magnetic-intensity Data

Reprocessing of seismic-reflection data reveals new images of upper- to middle-crustal structures beneath the Wabash Valley seismic zone, located north of the New Madrid seismic zone within the seismically active southern Illinois basin. Four intersecting deep seismic profiles (243 km total) indicate an anomalous, 5–10-km-wide zone of dipping reflections and diffractions below the western flank of the Wabash Valley fault system (WVFS). The zone corresponds in places to gently arched regions of Paleozoic strata. The reflector zone can be interpreted as a result of either (or a combination of) magmatic intrusion or structural deformation. The area encompassing the reflection profiles has experienced several moderate magnitude (3.0 ≤ mbLg ≤ 5.5) earthquakes during the past 50 years, defining the central part of the Wabash Valley seismic zone. The hypocenter of the largest 20th-century earthquake in the central USA midcontinent (9 November 1968, mbLg 5.5) corresponds to the most prominent zone of dipping middle-crustal reflections, just west of the WVFS. Both the focal mechanism (moderately dipping reverse fault) and the expected rupture zone size (∼2.9 km fault length) of this earthquake are consistent with the orientation and size of observed reflectors. Dipping reflector patterns in the Precambrian crust are not collinear with fault surfaces updip in the Paleozoic sedimentary section. This indicates that shallow Paleozoic structures are effectively “decoupled” from deeper, possibly seismogenic structure, which suggests that understanding Paleozoic structure is not the key to understanding the earthquake source. The complex dipping crustal reflectivity beneath the WVFS is typical of Paleozoic continental convergent zones observed elsewhere ( e.g. , Appalachian orogen) and thus may suggest a preserved Proterozoic suture, possibly associated with the distal Grenville orogeny or an older event. Although magnetic intensity, Bouguer gravity, and seismic-reflection data present different means of understanding the deep geology of the area, their integration aids in limiting the number of admissible interpretations. The reflection profiles indicate a variable zone of anomalous crustal structure, including the dipping reflector zone, along a trend of northeast-trending gravity and magnetic highs locally defining the Commerce geophysical lineament (CGL), which is a suspected source of seismic hazard in the central USA midcontinent. Three-dimensional inverse modeling of the residual isostatic gravity anomaly values indicates that the upper part of the dipping reflector zone beneath the CGL lies near an important density boundary in the upper Precambrian crust. The results of our study suggest that the seismogenic source just north of the New Madrid seismic zone consists, in part, of a pre-existing fabric of blind thrusts localized along pre-existing igneous intrusions, locally coincident with the CGL. This suggests that the CGL may be seismogenic in places and thus a potential seismic hazard. The variation in the expression of the CGL using reflection and potential-field data sets is probably partly related to the differing geologic features by which it is expressed, but would be also consistent with its reactivation numerous times under varying stress regimes.

Seismic-reflection profiles of the central part of the Clarendon–Linden fault system of western New York in relation to regional seismicity

Geological and geophysical research in upstate New York, with few exceptions, has not definitively associated seismicity with specific Proterozoic basement or Paleozoic bedrock structures. The central part of the Clarendon–Linden fault system (CLFS) between Batavia and Dale, NY is one of those exceptions where seismicity has been studied and has been spatially associated with structure. The CLFS is either a complex system of long faults with associated shorter branches and parallel segments, or a region of many short faults aligned north–south from the Lake Ontario shore southward to Allegany County, NY. Interpretation of 38 km of Vibroseis and approximately 56 km of conventional seismic-reflection data along 13 lines suggests that the CLFS is a broad zone of small faults with small displacements in the lower Paleozoic bedrock section that is at least 77 km long and 7–17 km wide and spatially coincident with a north-trending geophysical (combined aeromagnetic and gravity) lineament within the basement. The relative offset across the faults of the system is more than 91 m near Attica, NY. The CLFS is the expression of tectonic crustal adjustments within the Paleozoic rock above the boundary of two basement megablocks of differing petrologic provinces and differing earthquake characteristics that forms the eastern side of the Elzevir–Frontenac boundary zone. Deep seismic-reflection profiles display concave-eastward listric faults that probably merge at depth near the mid-crustal boundary layer. An interpretive vertical section provides the setting for refined definitions of the CLFS, its extensions at depth and its relation to seismicity. Most modern seismicity in western New York and the Niagara Peninsula of Ontario occurs in apparent patterns of randomly dispersed activity. The sole exception is a line of seven epicenters of small earthquakes that trend east from Attica, NY into the Rochester basement megablock. Earthquakes may be triggered at the intersections of north- and east-trending brittle faults within the Niagara basement megablock. Current interpretations of the mechanisms for earthquake generation in western New York and the Niagara Peninsula of Ontario require conservative estimates of seismic hazards that assume that an earthquake the size of the 1929 Attica, NY, event ( M b=5.2) or larger could occur anywhere in the Eastern Great Lakes Basin (EGLB). The broad zone of small-displacement faults that marks the CLFS in the lower Paleozoic sedimentary section and the uppermost basement may not provide the structural environment for generation of earthquakes in western New York. If this interpretation is correct, most seismicity is generated within the Niagara basement megablock beneath or west of the CLFS. Consequently, we may have to look to the deeper tectonic regime of basement megablocks to understand the distribution of modern seismicity in the EGLB.

Geology of the Subsurface Igneous Rocks of Sunflower County, Mississippi

ABSTRACT Mesozoic, Paleozoic, and Precambrian igneous rocks have been penetrated in the subsurface of northwestern Mississippi by the Pan American (Amoco) No. 1 Word, Sunflower County. This well encountered Late Cretaceous waterlain volcaniclastics, Late Jurassic trachytes, Middle to Late Ordovician basic intrusives, and Late Precambrian syenite. The Cretaceous volcaniclastic sequence occurs 85 m below the top of the Eutaw Formation and is comparable to rock types penetrated in northwest Mississippi dated at 78.3 to 91.3 Ma. The Jurassic igneous rocks comprise an 805 m trachyte flow complex 270 m below the top of the Cotton Valley Formation and were dated at 174 ± 8 Ma. Macroscopically, they are gray-green to gray-black, very fine to finely crystalline, slightly porphyritic, amygdaloidal flow rocks which have been severely altered and locally intruded by Cretaceous-Age(?) dikes. The Ordovician igneous section is apparently restricted to basic igneous intrusives in the Precambrian syenite and has been dated at 468 ± 19 Ma. The deepest igneous sequence encountered by the No. 1 Word is a 306 m interval of white to greenish-gray, fine- to medium-grained Precambrian syenite dated at 785 ± 34 Ma. The Precambrian section apparently formed during the rifting of North America and possibly represents the peripheral cratonic margin of Paleozoic North America. Tectonic uplift and erosion at the end of the Carboniferous Ouachita Orogeny removed the Paleozoic sedimentary section. The sole relict of this sequence is believed to be an 18 m conglomeratic quartzite of unknown age which lies between the Jurassic and the Precambrian. The Jurassic flows emanated from volcanic features associated with the Mesozoic igneous complex of the Monroe-Sharkey uplift. Several eruptive phases are apparent from petrographic and seismic evidence. This section was exposed to subaerial erosion only briefly before burial by younger Jurassic sediments. Between the Jurassic trachyte and the Cretaceous volcaniclastics are 878 m of sedimentary rocks of the Cretaceous Tuscaloosa Formation, undifferentiated Lower Cretaceous strata, and upper Jurassic Cotton Valley Formation. The Cretaceous volcaniclastics apparently reached the surface as tuffaceous material, were weathered, and redeposited to form the unit preserved today. In conclusion, all of this igneous activity has been concentrated in northwest Mississippi at the confluence of the Precambrian cratonic rift margin, the Paleozoic - Mesozoic intracratonic Reelfoot Rift, and the Mesozoic Gulf of Mexico peripheral fault zone. Further, this tectonism has created important structural and stratigraphic traps that should be excellent exploration objectives.

Paleomagnetism of Proterozoic and Cambrian Strata, Adrar de Mauritanie, CRatonic West Africa

Four paleomagnetic poles are reported from red beds of the Adrar de Mauritanie, cratonic West Africa. Thirteen localities were occupied, and 224 samples were collected in seven units from a middle Proterozoic to early Paleozoic sedimentary section. Two poles are reported from middle and late Proterozoic strata; a third pole is reported from Cambrian strata. The magnetizations for these reside in stable high‐temperature components, revealed by progressive thermal demagnetization analysis to 675°C. The fourth pole is interpreted to reflect a Permian overprinting in the Cambrian strata. Two Cambrian‐Ordovician units sampled contained directions too scattered to allow the calculation of any pole. The pole from middle Proterozoic strata (998±32 Ma; Rb‐Sr date on argillite), derived from four localities (52 samples, single polarity), plots at 48.7°N, 206.9°E (dp = 1.1°, dm = 2.9°). The pole from late Proterozoic strata (775 ± 52 Ma; Rb‐Sr date), derived from two localities (15 samples, mixed polarity), plots at 34.0°N, 246.9°E (dp = 4.6°, dm = 8.6°). Neither has correlatives in age in the paleomagnetic data set for cratonic West Africa, nor are there obvious correlatives with the paleomagnetic data set from the remainder of Africa. The pole that we consider to reflect a Cambrian direction comes from three localities (23 samples, mixed polarity), plotting at 0.3°S, 249.3°E (dp = 5.7°, dm = 11.5°). Its antiparallel pole corresponds with one group of Cambrian poles reported from Africa but differs markedly from another group of Cambrian poles. The late Paleozoic overprint direction obtained from three units (50 samples from four localities, single polarity) provides a (reversed polarity) pole that plots at 47.2°S, 42.6°E (dp = 1.5°, dm = 3.0°), which is in good agreement with Permian poles reported from elsewhere in Africa. In spite of the large number of poles already reported from Africa, additional data are needed before a reliable apparent polar wandering path can be constructed for the Paleozoic and Proterozoic of Africa.

40Ar/39Ar age of detrital muscovite within Lower Ordovician sandstone in the coastal plain basement of Florida: Implications for west African terrane linkages

Detrital muscovite was concentrated from a core of Lower Ordovician sandstone recovered from 1282 m in the Sun Oil Company, H. T. Parker No. 1 well, Marion County, Florida. The concentrate records a 40Ar/39Ar plateau age of 504.1±2.1 Ma. The Paleozoic sedimentary section penetrated in this well is part of an extensive subsurface Lower Ordovician-Middle Devonian sedimentary succession characterized by Gondwanan paleontological affinities. The succession has been correlated with sequences of similar age in the Bove Basin of west Africa which unconformably overlie metamorphic units of the Bassaride and Rokelide orogens in Senegal and Guinea. Muscovite within these metamorphic rocks records ca. 500–510 Ma postmetamorphic 40Ar/39Ar cooling ages and was likely a proximal source for the lower Paleozoic clastic detritus represented in the pre-Mesozoic sedimentary sequences beneath the southeastern United States coastal plain.

Intracrustal complexity in the United States midcontinent: Preliminary results from COCORP surveys in northeastern Kansas

Unusually clear indications of complex structure in the mid-to-lower crust is revealed by seismic reflection surveys in northeastern Kansas. This complexity contrasts markedly with the layer-cake simplicity of both the overlying sedimentary cover and most previous crustal models for the central United States. Seismic sections collected by COCORP (Consortium for Continental Reflection Profiling) as part of a major east-west traverse across the Neniaha Ridge and Midcontinent Geophysical Anomaly indicate that below a thin, relatively flat layered Paleozoic sedimentary section, the deep crust is characterized by numerous dipping and arcuate reflections and diffractions. In many places layered and crosscutting, these reflections suggest convoluted three-dimensional folded, faulted, and intruded structures. Specific identification of these deep features may be possible if future surveys can trace them to accessible depths. The basement above these reflection complexes contains significantly fewer reflections—consistent with, but not necessarily diagnostic of, the granitic terrane that dominates basement drill-hole samples in the region. Among the events at these shallower basement depths are several east-dipping reflections, some of which may be major faults. Travel times corresponding to expected Moho depths (about 36 km) are characterized less by specific reflections than by an apparent decrease in the density and number of reflections. While evidence of crustal heterogeneity is common among deep reflection studies, the Kansas seismic results outlined in this brief report stand out as being unusually clear representations of such.

Distribuição espacial e temporal das unidades litoestratigráficas paleozóicas na parte central da Bacia do Paraná

Paulipetro has drilled about a score of stratigraphic and wildcat wells in the Parana Basin getting unique informations of the interrelations and distributions of the rock units in its sedimentary filling. This paper presents the new stratigraphic chart demonstrating time-space relationships of the rock units and a revised chrono-stratigraphic classification for the Paleozoic sedimentary section of the Parana Basin. As an initial approach, the proposed chrono-stratigraphic column presents only three units (Serie Campos Gerais, Serie Rio Tiete and Serie Serra do Espigao). However, based on the distinctive geological evolution of the Parana Basin, it has essentially a regional character. The introduction or a new time-space diagram and simplified chrono-stratigraphic column are justified by the demand of improved stratigraphic correlations for petroleum exploration.

Pre-Quaternary Geology of North Greenland: ABSTRACT

In North Greenland a geologic section is present that contains Precambrian crystalline basement, and strata of Precambrian, early Paleozoic, late Paleozoic, Mesozoic, and Tertiary strata. The crystalline basement, which is exposed at places adjacent to the Inland Ice, is overlain with angular unconformity by the late Precambrian to early Paleozoic sedimentary section. These sediments have a gentle north dip and comprise a large platform area that extends from the west to the east coast. The oldest sedimentary formation of this platform (the Inuiteq S^phgr Formation) is at least 1,000 m.y. old, and the youngest strata are late Wenlockian-early Ludlovian. The lower Paleozoic strata, if traced northward, are part of the North Greenland fold belt, which occupies an approximately E-W-trending zone of folds and metamorphic rocks along the extreme northern coast of Greenland. In Peary Land, where the broadest section across the folded zone is exposed, metamorphic and deformat onal effects increase northward. In eastern Peary Land, the folded Cambrian, Ordovician, and Silurian sediments are overlain unconformably by less severely deformed Pennsylvanian, Permian, Triassic, and Cretaceous-Tertiary strata. This younger cover shows the effects of Tertiary earth movements. In northern Peary Land, a bedded sequence of dominantly rhyolitic lava and tuff (the Kap Washington Group) crops out. These volcanic rocks post-date the main Paleozoic diastrophism of the surrounding metasediments, but are affected by later folding and thrusting. A minimum K-Ar age of 35 m.y. has been obtained from the lavas. The metasediments of the North Greenland fold belt have been subjected to a complex structural and metamorphic history, which is not completely understood. Two distinct periods of deformation and metamorphism can be recognized: Paleozoic (between Late Silurian and Late Devonian times) and Cretaceous-Tertiary. Paleozoic orogenesis involved polyphase deformation in northern Peary Land with the second- and third-order folds facing northward, toward the assumed interior of the orogen. Cretaceous K-Ar ages of the metamorphic rocks suggest a subsequent thermal episode which produced Abukuma-type mineral assemblages, but no structural events can be assigned to this period. Tertiary movements are indicated by the northward thrusting of the metamorphic rocks over the Kap Washington Group, with accompanying mylonitization. End_of_Article - Last_Page 2476------------

Chronology of Deformation of Paleozoic and Tertiary Succession Near Railroad Valley, Nevada: ABSTRACT

Principal structures occurring in the Paleozoic sedimentary section of the Horse Range are thrust faults (some placing older over younger rocks, but most placing younger over older), north-south-trending, asymmetrical, eastward-overturned folds, and high-angle faults. Pre-Oligocene deformation of these Paleozoic rocks is indicated by Oligocene volcanic rocks lying with angular unconformity on overturned Ordovician strata. In the dissected pediments west of the Horse Range, a 10,000-foot-thick sedimentary and volcanic succession of Oligocene, Miocene, and Pliocene rocks crops out (Moores, 1965). The Miocene-Pliocene part of the section lies disconformably on Oligocene volcanic rocks and consists of an assemblage of terrestrial strata, including indurated ash beds, lacustrine limestone, and immature detrital deposits which contain angular Paleozoic carbonate and Oligocene volcanic clasts derived from adjacent ranges. The general parallelism of the Paleozoic and Tertiary successions given as evidence for contemporaneous deformation (Moores, 1965) is expressed only in that they are in contact with one another for a distance of approximately 12 miles. There the parallelism ends. Folds and thrust faults in the Paleozoic rocks trend north-south, whereas folds within the Tertiary succession trend east-west and plunge steeply toward the west. These relations, together with the angular unconformity between the Oligocene volcanic rocks and Paleozoic rocks, temporally separate tectonic deformation of the Paleozoic and Tertiary successions. Principal deformation of the Paleozoic succession at least preceded the Oligocene volcanic rocks, and may be as old as Pennsylvanian (Ptacek, 1963). Deformation of the Tertiary succession occurred, at least in part, after the Pliocene rocks were deposited, and probably during the Oligocene and Miocene, as is suggested by several unconformities reported in the Tertiary succession (Moores, 1965). Some occurrences of Paleozoic rocks are in positions that suggest emplacement by gravity sliding during or after deposition of Tertiary rocks. These occurrences can be explained easily as gravity-slide blocks emplaced from adjacent ranges (e.g. Horse and Grant) as the ranges were uplifted along high-angle faults, but in no way imply that the principal deformation of the Paleozoic strata was contemporaneous with deformation of the Tertiary succession. End_of_Article - Last_Page 650------------