Profound Unconformity Research Articles

Stratigraphic and tectonic implications of field and isotopic constraints on depositional ages of Proterozoic Lesser Himalayan rocks in central Nepal

In the Himalaya of central Nepal, uncertainty in the absolute depositional ages and the relative stratigraphic positions of several formations in the Proterozoic part of the Lesser Himalayan series has hindered structural mapping, east-west correlation of units and structures, and interpretations of basin architecture during both the Proterozoic and Paleozoic. At the stratigraphic base of the Lesser Himalayan series in central Nepal, 441 new U/Pb isotopic ages of detrital zircons from the previously defined type Kuncha, Kushma, and Fagfog formations constrain the maximum possible depositional ages for these clastic units to be c. 1900, 1770, and 1810Ma, respectively. 182 detrital zircon U/Pb isotopic ages confirm the identity of another sample of the Fagfog Formation, and field relations indicate that this unit rests depositionally on the Kushma Formation with no obvious unconformity or lithologic difference. This interpretation, combined with the recognition that the term “Kushma Formation” historically has been applied incongruously to apparently different units in different parts of Nepal, leads us to recommend abandoning the term “Kushma Formation” and replacing it with “lower Fagfog Formation.” The overlying part of the Fagfog then becomes the “upper Fagfog Formation.” Because the 1770Ma constraint on the maximum depositional age comes from the lower part of the unit, the entire Fagfog Formation must have been deposited after 1770Ma. The depositional ages of both the lower and upper parts of the Fagfog must be younger than the Kuncha Formation because it was intruded by a granite at 1878±22Ma (2-sigma). Thus an unconformity lasting at least 86M.y. probably separates the Kuncha Formation (deposited between 1900 and 1856Ma, considering uncertainties) and the Fagfog Formation (deposited after 1770Ma). The lower Fagfog Formation is present in the Kali Gandaki region but apparently pinches out to the east because it is not present structurally beneath the Kathmandu nappe.Suggestions about the depositional age of the carbonate-dominated Malekhu Formation at the top of the Proterozoic part of the Lesser Himalayan series have ranged from Mesoproterozoic to late Paleozoic. These age estimates largely stem from correlation with parts of the Krol succession some 500km to the west in northwest India, and changing interpretations of the depositional age of the Krol rocks. 104 new measurements of carbon isotope abundances in carbonate from a 300m measured section through type exposures of the Malekhu Formation reveal a narrow range of δ13C values between −1.7 and +0.2‰ (VPDB) and a mean of −0.9±0.4‰ (1 standard deviation). These values are inconsistent with correlation with the Krol succession, which preserves large amplitude positive and negative excursions in δ13C values. Comparison with known marine carbonate δ13C values through time suggests deposition of the Malekhu Formation prior to c. 1300Ma. Upper and lower bounds for deposition of the Proterozoic part of the Lesser Himalayan series thus are c. 1300 and c. 1900Ma, though actual deposition likely occurred during only a portion of this interval. Further, a profound unconformity representing at least 900M.y. separates the Malekhu Formation from the depositionally overlying Carboniferous-Permian Sisne Formation. No upper Mesoproterozoic, Neoproterozoic, or lower Paleozoic rocks have been found in the Lesser Himalayan series in central Nepal. The carbon isotope stratigraphy of the Malekhu Formation in central Nepal matches that of the Buxa Formation exposed in the Ranjit window of Sikkim to the east, but does not match the chemostratigraphy of rocks correlated with the Buxa Formation in eastern Bhutan and Arunachal Pradesh, northeast India.

The lower Lesser Himalayan sequence: A Paleoproterozoic arc on the northern margin of the Indian plate

The lower Lesser Himalayan sequence marks the northern extremity of the exposed Indian plate, and is generally interpreted as a passive margin. Five lines of evidence, however, collectively suggest a continental arc setting: (1) igneous intrusions and volcanic rocks occur at this stratigraphic level across the length of the Himalaya, (2) ages of intrusive and metavolcanic (?) rocks cluster at 1780–1880 Ma but also indicate a long-lived igneous process, (3) detrital zircon ages in clastic rocks cluster at 1800–1900 Ma, with a unimodal age distribution in some rocks, (4) the mineralogy and chemistry of metasedimentary rocks differ from typical shales and suggest a volcanogenic source, (5) trace-element chemistries of orthogneisses and metabasalts are more consistent with either an arc or a collisional setting. Intercalation of volcanic rocks with clastic sediments and a general absence of Proterozoic metamorphic ages do not support a collisional origin. An arc model further underscores the profound unconformity separating lower-upper Lesser Himalayan rocks, indicating that a Paleoproterozoic arc may have formed the stratigraphic base of the northern Indian margin. This, in turn, may indicate disposition of the Indian plate adjacent to North America in the ca. 1800 Ma supercontinent Columbia. Felsic orthogneisses (“Ulleri”) likely represent shallow intrusions, not Indian basement.

Sequence evolution in the eastern Chhattisgarh Basin: constraints on correlation and stratigraphic analysis

The Proterozoic succession in the eastern part of the Mesoproterozoic Chhattisgarh Basin comprises two unconformity-bounded sequences. Sequence I represents the Chhattisgarh Supergroup of earlier workers. It overlies rocks of the basement complex with a profound unconformity. Sequence II unconformably overlies Sequnce I, and represents the closing phase of basin evolution during the early Neoproterozoic time. It is unconformably overlain by rocks of the Gondwana Supergroup.
 The Lohardi and Gomarda formations at the lower part of the Chandarpur Group of Sequence I comprise an immature succession of conglomerate, sandstone and shale deposited in fan-delta - pro-delta environments, marked by rapid facies changes, variable rates of sediment influx, and uneven rates of subsidence and creation of accommodation space. The Kansapathar Sandstone in the upper part of the Chandarpur Group, by contrast, comprises a sheet of mature arenite deposited in a macrotidal shelf. The immature assemblage is best developed in the eastern part of the basin, and rapidly thins out towards west, where the Kansapathar arenite directly overlies the basement. The Raipur Group provides an excellent example of cyclic sedimentation of red shale and limestone. It comprises three shale-dominated intervals and two carbonate-dominated intervals, organized into multiple shallowing-up cycles. The lower carbonate succession, the Sarangarh Limestone, developed as a shallow water un-rimmed platform and evolved into a deep water ramp, with an extensive thin sheet of black limestone facies. Stromatolites are conspicuously absent in the Sarangarh Limestone. Small stromatolite bioherms appear in the Gunderdehi Shale which overlies the ramp succession, and abundant growth of stromatolite is noted in the upper carbonate succession which evolved as a rimmed platform. A thick ignimbrite horizon in the Churtela Shale attests to major felsic volcanism and termination of the Sequence at ~1000 Ma.
 The Kansapathar Sandstone, the black limestone facies of the Sarangarh Limestone, and the Gunderdehi Shale embedded with small stromatolite bioherms can be used as key marker horizons to overcome the problem of intrabasinal correlation. The marker horizons can be traced from the western part to the eastern part of the basin. The stromatolites in the Gunderdehi Shale and in the Saradih Limestone further provide a biostratigraphic frame, subject to detailed morphologic and microstructural analysis, for possible chronostratigraphic classification.

Stratigraphic and tectonic settings of Proterozoic glaciogenic rocks and banded iron-formations: relevance to the snowball Earth debate

Among Palaeoproterozoic glacial deposits on four continents, the best preserved and documented are in the Huronian on the north shore of Lake Huron, Ontario, where three glaciogenic formations have been recognized. The youngest is the Gowganda Formation. The glacial deposits of the Gowganda Formation were deposited on a newly formed passive margin. To the west, on the south side of Lake Superior, the oldest Palaeoproterozoic succession (Chocolay Group) begins with glaciogenic diamictites that have been correlated with the Gowganda Formation. The >2.2 Ga passive margin succession (Chocolay Group=upper Huronian) is overlain, with profound unconformity, by a >1.88 Ga succession that includes the superior-type banded iron-formations (BIFs). The iron-formations are therefore not genetically associated with Palaeoproterozoic glaciation but were deposited ∼300 Ma later in a basin that formed as a result of closure of the “Huronian” ocean. In Western Australia, Palaeoproterozoic glaciogenic deposits of the Meteorite Bore Member appear to have formed part of a similar basin fill. The glaciogenic rocks are, however, separated from underlying BIF by a thick siliciclastic succession. In both North America and Western Australia, BIF-deposition took place in compressional (possibly foreland basin) settings but the iron-formations are of greatly different age, suggesting that the most significant control on their formation was not oxygenation of the Earth’s atmosphere but rather, emplacement of Fe-rich waters (uplifted as a result of ocean floor destruction?) in a siliciclastic-starved environment where oxidation (biogenic?) could take place. Some of the Australian BIFs appear to predate the appearance of red beds in North American Palaeoproterozoic successions and are therefore unlikely to be related to oxygenation of the atmosphere. Neoproterozoic glaciogenic deposits are widespread on the world’s continents. Some are associated with iron-formations. Two theories have emerged to explain these enigmatic BIFs. According to the snowball Earth hypothesis (SEH), ice-covered oceans would have permitted buildup of dissolved Fe. Precipitation of Fe-rich sediments would have taken place following reoxygenation of the hydrosphere as the ice cover disappeared. A second theory involves glaciation of Red Sea rift-type basins. Fe-charged brines in such basins would have precipitated on being mixed with “normal” seawater as a result of glacially driven thermal overturn. Both theories provide an explanation of the hydrothermal imprint on the geochemistry of Neoproterozoic BIF but the restricted development of BIF (relative to glacial deposits), evidence of rift activity such as significant facies and thickness changes, and association with volcanic rocks, all favour deposition in a rift environment. Cap carbonates are one of the cornerstones of the SEH. Escape from the snowball condition is said to have resulted from buildup of atmospheric CO 2 while the weathering cycle was stopped. Under such conditions, the first siliciclastic deposits following glaciation, should be extremely weathered, and should be overlain by sedimentary rocks that show a gradual return to more “normal” compositions. Using a chemical index of alteration (CIA) it can be shown that, in the case of the Gowganda Formation, the CIA shows a gradual upward increase, opposite to that predicted by the SEH. The Earth underwent severe climatic perturbations both near the beginning and end of the Proterozoic Eon but whether it attained a totally frozen surface condition (as postulated under the SEH) remains speculative.

Palaeomagnetism and magnetostiatigraphy of Triassic strata in the Sangre de Cristo Mountains and Tucumcari Basin, New Mexico, USA

We report palaeomagnetic data and a composite magnetic polarity sequence for Middle and Upper Triassic rocks assigned to the Anton Chico Member of the Moenkopi Formation and Chinle Group, respectively, exposed along the eastern flank of the Sangre de Cristo Mountains and in the Tucumcari Basin of eastern and northeastern New Mexico. Thermal demagnetization isolates a well-defined, dual polarity, characteristic magnetization, carried in most cases by haematite and interpreted as an early acquired chemical remanent magnetization (CRM). Characteristic magnetizations from 74 palaeomagnetic sites (one site=one bed) are used to define a magnetic polarity sequence, which we correlate with previously published Triassic data obtained from both marine and non-marine rocks. Preliminary correlation suggests that the resolution of magnetostratigraphic data derived from continental strata is not necessarily of lesser quality than that from marine rocks. On the basis of the magnetostratigraphic data, a profound unconformity is believed to separate lower-middle Norian and upper Norian-Rhaetian strata of the Chinle Group. Palaeomagnetic poles derived from selected sites in steeply dipping (> 85°) strata for the Middle Triassic (Anisian, ≈240 Ma: 50°N 121°E; N=8), late Carman-early Norian (≈225 Ma: 53°N 104°E; N=16), and late Norian-Rhaetian (≈208 Ma: 59°N 77°E; N=8) are in relatively good agreement with previously published data for the Moenkopi Formation and Chinle Group and related strata in southwest North America. None the less, comparison with palaeomagnetic poles obtained from gently dipping or flat-lying Triassic strata from this study (Anisian, 46°N 112°E; N=13; late Carnian, 54°N 87°E; N=12) and previously published Triassic poles in southwest North America suggest that a modest ‘apparent rotation’ not greater than about 5° affects declinations from steeply dipping rocks. The distribution of palaeomagnetic poles indicates ≈25° (angular distance) of apparent polar wander between about 240 and 208 Ma.

Paleomagnetism of the late Neoproterozoic Blueflower and Risky formations of the northern Cordillera, Canada

Paleomagnetic analysis of the Risky and underlying Blueflower formations at the top of the Neoproterozoic Windermere Supergroup in the northern Cordillera of Canada revealed several magnetic components of geological significance. Rock strata of the Blueflower locality in the Mackenzie Mountains yielded no primary remanence due to extensive "pyritization" and weathering. However, dolomitic rocks of the Risky locality in the eastern Wernecke Mountains, 250 km to the north, yielded a probable primary remanence. The remanence has a direction at D, I = 055°, −64° (N = 11 samples from 4 sites, k = 18, α95 = 11°) and pole at 007°E, 29°S (δp, δm = 14°, 18°), and indicates a paleolatitude of 46 ± 14°S. Comparison between poles from the Risky and the overlying Backbone Ranges and Ingta formations suggests a profound post-Risky unconformity. Three secondary components are also present. Component B in both the Blueflower and Risky units appears to be a thermal overprint, possibly related to Vendian rifting. Its respective direction in each unit is D, I = 062°, +77° (N = 4 sites, k = 143, α95 = 8°) and D, I = 064°, +56° (N = 6 sites, k = 21, α95 = 15°), yielding poles of 110°E, 64°S (δp, δm = 13°, 14°) and 142°E, 44°S (δp, δm = 15°, 21°). Component C, only found in the Risky, is attributed to Late Cambrian to Middle Ordovician extension and magmatism. Component D, which predominates in both formations, relates to late Mesozoic compression and to weathering.

Evolution and significance of an overfilled alluvial foreland basin: Burnside Formation (1.9 Ga), Kilohigok Basin, N.W.T., Canada

AbstractThe Burnside Formation records the transition from marine shelf to largely alluvial conditions in the palaeo‐Proterozoic (about 1.9 Ga) Kilohigok foreland basin in the Slave Province of the Canadian Shield. The Burnside Formation forms a thick (up to 3.5 km), N W‐tapering siliciclastic wedge, representing a braided alluvial system that prograded transversely across the Slave Province. The stratigraphic architecture of this unfossiliferous, predominantly alluvial succession documents the location and distribution of unconformities caused by lithospheric flexure in a Proterozoic foreland basin. The locations of these unconformities demonstrate the changing locations of tectonic and sedimentary loads on both sides of the Silave craton.Associations of sedimentary facies and palaeocurrents indicate that initially a braid delta system drained longitudinally, and was restricted to the proximal part of the basin between the orogenic hinterland (the Thelon Orogen) and a syn‐sedimentary flexural arch. Palaeocurrents for the lowermost braid delta facies in the proximal foreland were parallel to palaeocurrents in underlying foreland basin turbidites, parallel to the flexural arch closest to the Thelon Orogen, parallel to the strike of the orogen, and perpendicular to the transport direction of thrust‐nappes directed out of the orogenic belt. An abrupt shift to a transverse palaeocurrent mode accompanied the onset of higher‐gradient braided stream conditions across the entire foreland basin, showing that from this time, sediment supply exceeded subsidence in the proximal foreland. The long‐lived, laterally extensive transverse dispersal and the presence of sedimentary facies lacking interbedded mud or sedimentary structures indicative of desiccation suggest that fluvial discharge was probably perennial.The sharp superposition of medial braid plain facies directly on marine shelf and deltaic facies in the distal foreland indicates that the transition to non‐marine conditions marks an erosional unconformity over two coeval flexural arches. The eastern flexural arch relates to covergence on the eastern margin of the Slave Province and to thrust loading in the Thelon Orogen that created the Kilohigok foreland basin. The evolution of a passive margin on the western side of the Slave Province caused the other flexural arch. In the centre of Kilohigok Basin a profound unconformity exists at the top of the Burnside alluvial succession. This unconformity suggests that a single flexural arch existed at the end of alluvial sedimentation that may signify the migration and yoking together of the two previously independent flexural arches. This migration may have been caused by the combined mass of the thick alluvial sedimentary wedge over the eastern edge of the Slave craton, coupled with initial thrust loading and foreland basin development in Wopmay Orogen.The transition within a foreland basin from largely marine to largely alluvial deposition reflects changing subsidence patterns across the basin due to changes in rates of convergence and changing subsidence and sedimentation patterns due to erosional mass redistribution from the orogenic hinterland into the foreland basin. This transition appears to separate two distinct phases in the evolution of the Kilohigok foreland basin. In the first phase, active thrust loading and asymmetric flexural subsidence restricted alluvial facies near the orog;en. In the second phase, convergence slowed or ceased, and erosional unloading and uplift of the hinterland caused subsidence patterns to become more uniform across the foreland basin, promoting rapid progradation of alluvial facies across the basin. Palaeomagnetic and sedimentological data indicate that the basin lay in the trade wind belt and that humid or monsoonal climatic conditions existed at the time. This suggests that moist trade winds blew onto the Thelon Orogen on the side closest to the Kilohigok Basin. Such an orographic effect would have caused rapid erosion, localized the position of the Thelon orogenic front, increased the flux of material through the orogenic belt, and caused more extensive progradation of the alluvial system.

The geology and tectonic evolution of the Pietersburg Greenstone Belt, South Africa

The Pietersburg Greenstone Belt occurs near the northern margin of the “low-grade” granite-greenstone terrain of the Kaapvaal Craton. To the north lies the “high-grade” granite-greenstone terrain which constitutes the Southern Marginal Zone of the Limpopo Belt. The original relationship between the two terrains has been overprinted by the effects of the Limpopo Orogeny and are therefore not known. A better understanding of the tectonic evolution of the Pietersburg Greenstone Belt may help to clarify this relationship. The rocks of the southwestern part of the Pietersburg Greenstone Belt have been mapped in considerable detail. The rocks can be subdivided into two sequences. The first comprises a sequence of massive to pillowed metabasalts, metagabbros and metaperidotites, as well as BIF, fine-grained mafic tuffs and sediments. The sequence was subjected to extensive hydrothermal metamorphism at low pressures and variable temperatures in the greenschist to amphibolite facies. This suggests an oceanic-like environment. The second sequence, known as the Uitkyk Formation, comprises a sequence of terrestrial clastics, which overlie the first sequence with a profound unconformity. Sedimentology indicates that the Uitkyk Formation formed in a prograding alluvial fan or braided alluvial plain. The Uitkyk sediments were deformed and infiltrated by fluids that precipitated quartz-tourmaline (±gold) and siderite soon after deposition. Age relationships, bedding/cleavage relationships and younging directions indicate that both the sediments and simatic rocks were involved in a phase of thrust-related deformation (D 2) that resulted in south over north stratigraphic repetition under generally low-grade conditions of metamorphism. Post-D 2 granites date this thrusting at pre 2.69 Ga. D 2 was probably diachronous, and at surface, the deformation was accompanied by syntectonic terrestrial sedimentation and placer gold deposition. The tectonic environment was probably one of an intermontane accretionary or terrestrial foreland basin. Within this tectonic environment, the basement and cover became tectonically stacked across evolving shear zones. Combined field and microstructural relationships within a major fault zone have revealed a complex history of progressive D 2 deformation, metamorphism, mineralisation and associated sedimentary processes. The zone represents a deformed syn-sedimentary duplex that was carried piggy-back above an allochthonous thrust sheet of simatic rocks. At the deepest levels this fault, and other shear zones, were intruded by syntectonic granitoids. This late-Archaean, northward-verging D 2 thrust-stack model for the greenstone belt, with the syntectonic granites intruding the thrusts from beneath the greenstone belt, is in good accord with the recent geophysical observations. The age of cover sedimentation is older than ∼2.69 Ga but younger than 2.98 Ga in age; this supports suggestions that these deposits are age equivalent of the upper, gold-bearing sediments of the Witwatersrand basin; and that the northwards-thrusting in both basins may be of the same age and related to the same regional tectonic forces. The simatic rocks of the Pietersburg Greenstone Belt may have been part of a more extensive allochthonous simatic terrain, which includes the “similar” greenstone remnants of the Southern Marginal Zone of the Limpopo belt. Such a terrain must have been disrupted and metamorphosed during the northward-directed imbrication and tectonic burial recorded in the Pietersburg Belt, prior to the climax of the Limpopo Orogeny. The latter subsequently severely disrupted the Pietersburg Greenstone Belt during Ventersdorp times (2.6–2.7 Ga).

Proterozoic age of the Thule Group: new evidence from microfossils

This report deals with recent results of a micropalaeontological programme aimed at a biostratigraphic dating of the Thule Group. Earlier results are reported on by Vidal & Dawes (1980). The Thule Group is a several kilometre thick, sedimentary and volcanic sequence that forms large areas of North-West Greenland where it overlies the Archaean-Proterozoic crystalline basernent with profound unconformity (fig. 7). The strata are unmetamorphosed and they form flat-Iying to shallow-dipping outcrops; faulting, crushing, minor folding, and contact 'metamorphic' effects due to basic igneous intrusion, are the only post-depositional disturbances to have affected the sequence. Acritarchs are generally well preserved and, with the exception of rocks affected by contact metamorphism in connection with emplacement of basic rocks, indicate a low index of thermal alteration.

Late Paleozoic Cold-Water Carbonate Sedimentation in Tasmania: DISCUSSION

We read the recently published account of Tasmanian Late Paleozoic cold-water carbonate sedimentation (Rao, 1981) with considerable dismay. It is not that we disagree with a cold-water origin for the Berriedale Limestone (and other Tasmanian Late Paleozoic rocks), for this has long been the view of most Tasmanian geologists. Rather we indicate below what we consider to be fundamental errors in Dr'. Rao's understanding of the stratigraphy of Tasmanian Late Paleozoic rocks, and their bearing on what we consider to be a totally untenable model for Berriedale Limestone deposition. In Tasmania rocks of Late Paleozoic age are widespread and display an unmistakable glacial imprint. They constitute the Lower Parmeener Super-Group (Forsyth et al., 1974) which is ahnost everywhere subhofizontal, and rests with profound landscape unconformity on a folded basement of Precambfian and earlier Paleozoic strata intruded by Late Devonian to Early Carboniferous granites (McDougall and Leggo, 1965). Basement surfaces are often scoured, plucked and striated, with exhumed roches moutonndes in rare instances. The subglacial surface had a relief in excess of one thousand meters. Vestiges of this relief remained until their final burial late in the Triassic Period. Without doubt, sometime late in the Carboniferous Period ice covered most of Tasmania. When the ice retreated, thick sequences (sometimes in excess of 500 meters) of massive and unbedded mixtite, tillite, glaciolacustrine rhythmite claystane, outwash conglomerate, and minor turbidite sandstone were deposited in areas of low relief within the basement (David, 1908; GuUine, 1977; Banks, 1981). In the light of recent recommendations concerning the placement of the Carboniferous-Permian boundary (Kemp et al., 1977; Balme, 1980), all these glacigene rtx:ks are wholly Late Carboniferous in age (Truswell, 1978). Following the major ice retreat, most of Tasmania was rapidly flooded by a transgressive sea from the south and east, the initial eustatic rise in sea-level being far more rapid than any isostatic readjustment. Thereafter Tasmania suffered repeated periods of transgression and regression throughout Asselian-Kazanian times, when several hundreds of meters of shallow-water and essentially marine platform sediments were deposited. No subsequent transgression was as widespread areally as the initial post-glacial flooding. Dropstones and lonestones are ubiquitous and can only be satisfactorily explained by ice-rafting. Very rich faunas of very low taxonomic diversity (Clarke and Banks, 1975; Clarke and Farmer, 1976) support the existence of cold-water conditions. Nevertheless, there is no evidence that any part of what is now Tasmania was occupied by ice following the major

Extension of the telluride erosion surface to Washington State, and its regional and tectonic significance

Abstract The Wenatchee Formation of Oligocene age (34 m.y.) rests on a profound unconformity that developed during an extended period of erosion and tectonic quiescence from perhaps 40–34 m.y. B.P. Its occurrence strongly suggests that a stripped erosion surface in the northern Cascade Range, previously considered of Miocene age, is of Late Eocene—early Oligocene age and that it is part of a more widespread surface in the Pacific North-west. The Late Eocene—early Oligocene unconformity/erosion surface is apparently of continental extent, from British Columbia to Mexico. Where it is incised on crystalline basement rocks, it now may be exhumed and exposed as a plateau or level-crested divides of concordant elevation. Where it is developed on sedimentary rocks, it may be found as a profound unconformity. Widespread eruptions of pyroclastic volcanic rocks covered much of the surface in Oligocene time and tended to protect and preserve it. The surface is particularly well documented in the San Juan volcanic field of Colorado. There it was given the name “Telluride peneplain”, which is here broadened to the Telluride erosion surface of continental extent. A magmatic gap indicating a period of magmatic quiescence is generally associated with tectonic quiescence implied by the erosion surface, but the timing varies regionally. Development of the Telluride surface and associated magmatic gaps correlates with general plate reorganization during the Eocene. An erosion surface correlates with a global tectonic transition because erosion rates immediately dominate uplift rates when compressive deformation is slowed or shut off. An analysis of the dynamic balance between erosion rates and plate convergence rates suggests that erosion may maintain a continental surface of low relief if the relative convergence rate falls to about 0.5 cm/yr or less. Gaps in magmatic activity do not correlate as precisely with the 40 m.y. tectonic transition, in part because of the possibility of time lags. A great deal of scattered geologic evidence suggests a major global event at about 40 m.y. BP. The timing does not always precisely correlate with the timing of events in western North America, which suggests that the 40 m.y. tectonic transition was spread over several millions of years: A most impressive global impact of the 40 m.y. transition is the largest drop in eustatic sealevel at about 30 m.y. BP. The 40 m.y. tectonic transition profoundly altered geologic history, including the development of continental landscapes, global sea-level, and global climate. Classical stratigraphic studies remain the best way of documenting its effect on the continental surface. The small remnant of Oligocene rocks in central Washington, overlooked in early reconnaissance studies, is of great geologic significance. Other undiscovered remnants probably exist in western North America.

Permian and Pennsylvanian Stratigraphy, Disconformities, Facies Changes and Structure, Kananaskis and Spray Lakes Areas, Southern Canadian Rocky Mountains

ABSTRACT Detailed correlation of sections of Pennsylvanian and Permian sequences in the central Front Ranges reveals spectacular lateral facies changes, thickness variations, discontinuities, diamstems, and paraconformities, as well as disconformities with silcrete surfaces, basal conglomerates, regolith-like novaculitic cherts, bioturbated surfaces and local erosional relief up to 5 ft (1.6 m). Such disconformities can represent regional, very low angle truncations as evidenced by the profound unconformity developed within the Permian below the Ranger Canyon Formation (Guadalupian). A thrust fault displaying a localized lateral ramp occurs in the northern Elk Mountains, and the Permian and Pennsylvanian successions change facies across the fault.

Acritarchs from the Proterozoic Thule Group, North-West Greenland

A thick sequence of Proterozoic sedimentary and volcanic rocks (the Thule Group), penetrated in part by basic minor intrusions, characterises the Thule district of North-West Greenland (fig. 6). These strata, overlying the Greenland Shield with profound unconformity, form a large intracratonic basin on Ihe northern margin af the North American Greenland craton (Dawes, 1976).

Eastern Cordilleran Foldbelt and Foreland of Northern Canada: ABSTRACT

The eastern Cordilleran foldbelt and foreland of Canada north of lat. 60°N lies between the Tintina trench and the Canadian shield. It includes part of the southern rim of Canada basin and the junction between the Cordilleran and Innuitian orogenes. The sedimentary succession rests with profound unconformity on the westward continuation of the crystalline rocks of the shield, tapering eastward to a zero edge against the shield but truncated northward at the outer edge of the continental shelf. Widespread unconformities within and beneath the succession attest to regionally episodic orogeny and epeirogeny from the Proterozoic into the Tertiary. In contrast to the tectonic style of equivalent thrust-faulted parts of the southern Canadian Cordillera, the region is characterized by bundles of right- and left-hand en echelon folds cut by two major, right-lateral strike-slip fault systems, the Richardson fault array bordering upon the foreland, and the Kaltag fault zone transecting the foldbelt. Laramide horizontal shortening with concomitant vertical thickening of the sedimentary succession is roughly one-fourth that at the 49th Parallel. Known reservoir rocks for hydrocarbons include lower Paleozoic platformal carbonate rocks just north of lat. 60°N in both the foreland (Rabbit Lake) and eastern margin of the foldbelt (Beaver River and Pointed Mountain); Middle Devonian reefs (Norman Wells) and lower Carboniferous sandstone traps (Chance) within the foldbelt; and most recently, shale-cored anticlines and growth-fault structures in the Tertiary clastic sequence on the continental shelf at the junction of the Cordilleran and Innuitian Orogens. End_of_Article - Last_Page 759------------

Separation of Morocco and eastern North America: A Triassic-Liassic stratigraphic record

New primary data from northwest Africa show that the lower Mesozoic rocks of Morocco rest with profound unconformity on Hercynian metamorphic and/or Autunian sedimentary rocks and occur in three distinct, partially synchronous volcanic-sedimentologic provinces: the Oran Meseta, the High Atlas, and the Moroccan Meseta. The Oran Meseta of northwestern Morocco contains a Middle to Late Triassic andesite and carbonate-evaporite facies related genetically to the Tethys basin. The High Atlas province of southwestern Morocco consists of Late Triassic to Early Jurassic red beds and evaporites interbedded with tholeiite lavas. These tholeiites are underlain by the Minutosaccus - Patinasporites Concurrent Range Zone of middle Carnian age and yield an average isotopic age of about 196 m.y. They are time- and rock-stratigraphic correlatives of the First and Second Watchung-York Haven suite and the Quarryville basalts of Pennsylvania and New Jersey. The Moroccan Meseta of western and central Morocco consists of Lower Jurassic (Liassic) evaporites intercalated with low-alkali quartz tholeiites, yielding an isotopic mean age of 186 ± 8 m.y. This tholeiite is a rock- and time-stratigraphic correlative of the Rossville basalt of Pennsylvania. The tectonic model best explaining the chemical and stratigraphic distribution of lower Mesozoic rock now on the margins of the North Atlantic includes the following sequence: (1) Permian to Late Triassic uplift and crustal thinning along the axis of the future Atlantic Ocean; (2) Middle to Late Triassic strike-slip faulting and andesitic volcanism along east-trending fracture zones, followed by a westward advance of the Tethys Sea across northern Morocco; (3) Late Triassic rifting along the axis of the proto-Atlantic Ocean and shearing along east-west fracture zones, which had the combined effect of decoupling segments of the African and North American plates and providing a pathway for the marine transgression of the Tethys Sea across northern Morocco and south along the axis of rifting; and (4) Late Triassic to Early Jurassic crustal extension and extrusion of olivine and quartz tholeiites, followed by extrusion of subalkalic quartz tholeiites and collapse of the continental margins with the concomitant deposition of marine carbonates.

Mechum River Formation: Late Precambrian (?) Alluvium in the Blue Ridge Province of Virginia

ABSTRACT The Mechum River Formation is a thick succession of metamorphosed conglomerate, sandstone, siltstone, and shale exposed within a narrow, elongate structural infold (the Batesville syncline) along the axis of the Blue Ridge anticlinorium in central Virginia. The thickness of the unit is variable, ranging from approximately 1500 to 3000 ft. Stratigraphic position suggests that the Mechum River Formation may be at least partly correlative with the Swift Run and Lynchburg Formations, but there is no specific faunal evidence for age or correlation. Sedimentary petrology indicates that the unit was derived from uplifted parts of the igneous and metamorphic rocks of the Virginia Blue Ridge Complex which the Mechum River Formation overlies with profound unconformity. A variety of depositional environments (alluvial-fan?, channel fill, point bar, braided stream, and floodplain facies) can be inferred for the unit on the basis of texture and vertical succession of sedimentary structures, but it is not possible to specifically interpret the relative importance of each of these nonmarine environments. Crossbedding azimuths from the southeastern margin of the unit point southwest, those from the northwestern margin point south and southeast. This crudely centripetal paleocurrent system may reflect the existence of an elongate basin extending northeast-southwest. Such a basin may relate to the separation of North America from Africa and the opening of the Paleozoic Protoatlantic Ocean. However postulating such a basin solely on the basis of the evidence in this paper would be speculative.

Structures in the Iranides of South-eastern Turkey

The effects of two phases of deformation in Cretaceous rocks of the Alpine Orogenic Zone of south-eastern Turkey are described. These can be correlated with the late Cretaceous and Tertiary phases of deformation already known from the zone of foreland folding to the south. Some details of the stratigraphical succession are given and a previously unrecorded development of Oligocene rocks is described. The upper Cretaceous sediments are dominantly marine carbonates but include a red bed formation. Basic volcanics are intercalated with the lower carbonate beds. A profound unconformity separates the Eocene from the Upper Cretaceous. This unconformity results from uplift consequent upon the first phase of deformation which produced north-westward plunging folds and related thrusts. Some of these north-westerly folds have been considerably modified by the superposition of younger east-north-easterly trending Alpine folds and thrusts.

Geochronology of stratified rocks of the Canadian Shield

In the Canadian Shield, four orogenies, each followed by a profound unconformity, serve to divide the succession into five natural time-stratigraphic units. These are named for convenience in mapping, description, and discussion. Rock units in type areas, rather than abstract time, form the basis for the classification. Isotopic dates place the geological units in a scale of years and are extremely valuable for the purpose of long range correlation.

Stages and epochs of mineralization in the San Juan Mountains, Colorado, as shown at the Dunmore Mine, Ouray County, Colorado

The Dunmore lode is localized along a persistent fissure zone over two miles long and averaging nearly 100 feet in width along the length of the Dunmore claim. The fault in which the lode is located offset the pre-Cambrian quartzite and slate walls about 4,500 feet prior to deposition of the overlying thick San Juan tuff. Late Tertiary faulting extended the fissure into the San Juan tuff, dropping the south side about 80 feet. Although most of the lode crops out in the pre-Cambrian rocks, it extends upward across the profound unconformity into the tuff. The lode is complex, containing sharply marked fissure veins, sheared slate dragged into the fault zone, a sheared dike, breccia chimneys, and pebble dikes. An assemblage consisting of quartz, sericite, kaolin, and pyrite appears to be of early Tertiary age. The more productive part of the lode, however, is late Tertiary and consists of a hematite chimney inclosing a copper shoot, a tungsten chimney, and compound base-metal fissure veins. These chimneys and veins are mostly localized along early Tertiary structures, but their mineral sequences correlate them with the second stage of late Tertiary deposition recognized by Burbank in the adjoining Red Mountain and other districts. The Dunmore lode, however, is believed to have been deposited at greater depth and higher temperature and nearer to a source of supply than the lodes of these districts. In structure as well as mineral assemblages the Dunmore lode shows a connection between the chimneys and veins of the region.

Franciscan-Knoxville Problem

Franciscan-Knoxville Problem