Lithic Conglomerates Research Articles

The Capiru Group: From passive margin to syn-orogenic units, implications for the evolution of the northern margin of the Curitiba microplate, Western Gondwana

The Capiru Group (Neoproterozoic) is a 4 km-thick succession deposited on the northern margin of the Curitiba microplate, Southern Ribeira Belt, Brazil. The Capiru Group successions are deformed within the Brasiliano - Pan African Ribeira Belt, which culminated in the amalgamation of Western Gondwana. Even though the rocks are metamorphosed and deformed, preserved sedimentary structures and outcrop exposures allowed the interpretation of depositional environments and stratigraphic analysis. The Capiru Group metasedimentary rocks were studied by field mapping, measured sections and bulk-rock chemical compositions. Six formations were identified, termed from oldest to youngest as: Santana (proximal to distal marine shelf), Juruqui (proximal to distal deltaic), Rio Branco (carbonate platform), Morro Azul (lagoonal to shallow marine), Morro Grande (proximal to distal deltaic/marine) and Bocaina formations (estuarine to shallow-marine). Two main basin stages were identified: a passive continental margin stage, with increasing tectonic quiescence followed by a marine regression, and a syn-orogenic stage at the Curitiba microplate margin.Chemical and mineralogical Fe- and Al-rich compositions suggest continental denudation during the passive continental margin stage, with low sediment input rates from a relatively low-gradient relief, highly weathered continental source, and stability since Mesoproterozoic time. Sedimentological and stratigraphic similarities were found among Capiru Group passive margin successions and units deposited on the Río de Plata and Southern Paranapanema margins.The paleoenvironmental interpretations, such as greenhouse conditions and high weathering rates alternated with depositional gaps, indicate climatic changes during the Capiru Group deposition, with far-field or Phantom Glacial influence related to the regional discordances. These features are important for the paleogeographic reconstructions that must consider a tropical or equatorial position ∼650 - 635 Ma for the Curitiba microplate.During deposition of the Bocaina Formation, the arkosic composition of sandstones and lithic conglomerate lags suggest source rejuvenation during the syn-orogenic stage, prior to the collisional thrusting that led to the Western Gondwana assembly.

Thermochronology in South America

The Capiru Group (Neoproterozoic) is a 4 km-thick succession deposited on the northern margin of the Curitiba microplate, Southern Ribeira Belt, Brazil. The Capiru Group successions are deformed within the Brasiliano - Pan African Ribeira Belt, which culminated in the amalgamation of Western Gondwana. Even though the rocks are metamorphosed and deformed, preserved sedimentary structures and outcrop exposures allowed the interpretation of depositional environments and stratigraphic analysis. The Capiru Group metasedimentary rocks were studied by field mapping, measured sections and bulk-rock chemical compositions. Six formations were identified, termed from oldest to youngest as: Santana (proximal to distal marine shelf), Juruqui (proximal to distal deltaic), Rio Branco (carbonate platform), Morro Azul (lagoonal to shallow marine), Morro Grande (proximal to distal deltaic/marine) and Bocaina formations (estuarine to shallow-marine). Two main basin stages were identified: a passive continental margin stage, with increasing tectonic quiescence followed by a marine regression, and a syn-orogenic stage at the Curitiba microplate margin.Chemical and mineralogical Fe- and Al-rich compositions suggest continental denudation during the passive continental margin stage, with low sediment input rates from a relatively low-gradient relief, highly weathered continental source, and stability since Mesoproterozoic time. Sedimentological and stratigraphic similarities were found among Capiru Group passive margin successions and units deposited on the Río de Plata and Southern Paranapanema margins.The paleoenvironmental interpretations, such as greenhouse conditions and high weathering rates alternated with depositional gaps, indicate climatic changes during the Capiru Group deposition, with far-field or Phantom Glacial influence related to the regional discordances. These features are important for the paleogeographic reconstructions that must consider a tropical or equatorial position ∼650 - 635 Ma for the Curitiba microplate.During deposition of the Bocaina Formation, the arkosic composition of sandstones and lithic conglomerate lags suggest source rejuvenation during the syn-orogenic stage, prior to the collisional thrusting that led to the Western Gondwana assembly.

A review and new descriptions of Elk Point Group outcrops in the Athabasca Oil Sands mining region

Abstract Elk Point Group outcrops in the Athabasca Oil Sands mining region (AOSMR) and adjacent areas include exposures of the La Loche, Contact Rapids, Keg River, and Prairie Evaporite formations. Here, we review prior investigations of these formations in outcrop, followed by new descriptions of some outcrops, including those previously unpublished or newly discovered. The fluvial to marginal marine sandstone and conglomerate of the La Loche Formation, informally known as the granite wash, outcrops along the Clearwater River in Saskatchewan, where it is sandwiched between the Precambrian basement and the Contact Rapids Formation. In Alberta, the La Loche Formation is exposed at a locality along Whitemud Falls, where it directly underlies the Keg River Formation as a lithic sandstone and fills paleokarst crevices in the Keg River dolostone. From these two outcrops we recognize three facies in the La Loche Formation: regolith, lithic conglomerate, and lithic to arkosic sandstone. The marginal marine shale, silt, and dolomite of the Contact Rapids Formation outcrops in Saskatchewan at its namesake Contact Rapids, but is exposed only as a slumped bank of grey to greenish mud. We examined Keg River Formation dolostone from three outcrops along the Clearwater River at Contact Rapids in Saskatchewan and Whitemud Falls and Cascade Rapids in Alberta. We also describe an outcrop on the Firebag River in Alberta. From these outcrops, we recognize three general facies: bedded to laminated cryptalgal dolomitized bindstone (originating from an intertidal paleoenvironment), coral and stromatoporoid-bearing dolomitized floatstone to rudstone (originally a reef), and crinoid and brachiopod dolomitized floatstone (from off-reef or inter-reef areas). A newly recognized outcrop of the collapse residue from the dissolved Prairie Evaporite Formation occurs along the Clearwater River, where cobbles and boulders of breccia and other less soluble Prairie Evaporite rock weather out of the river bank between several sulfur springs.

River drainage reorientation during placer gold accumulation, southern New Zealand

Many alluvial placer deposits around the world occur in river systems that have been affected by tectonic events, causing drainage reorientation and severance of links between placers and their sources. This study documents tectonic rejuvenation of topography in the Otago giant placer goldfield, New Zealand, which has resulted in numerous river capture and drainage reorientation events. These events have induced changes to gold transport directions and numerous stages of separation of detrital gold from primary sources. Goldfield-wide reconstructions of drainage patterns through time are as yet only possible for Miocene–Recent, and numerous earlier drainage changes back to Cretaceous primary orogenic mineralisation are probable. Variations in basement lithologies permit auriferous gravel provenance determinations, facilitating paleodrainage pattern reconstruction and documentation of river capture events. River capture events and timing of these events for gold-bearing paleodrainages have also been documented using genetic divergences of populations of freshwater galaxiid fish that were isolated by drainage reorientation. Gold-bearing quartz pebble conglomerates had a southeastward drainage in the Miocene. This was disrupted in the Pliocene by mountain range uplift and gold placer recycling, with deposition of lithic conglomerates containing only minor gold placers. The most dramatic changes in gold transport directions occurred through the Quaternary, as antiformal ranges grew across the pre-existing drainages. Miocene and Pliocene placers were recycled with numerous local (1–10 km scale) changes in river directions and numerous capture events. Large axial rivers were segmented into a more complex drainage pattern, and on-going river capture resulted in growth of the main Clutha River catchment at the expense of neighbouring catchments. The most productive placers developed in the Clutha River in late Quaternary when increased discharge from captured mountain catchments enhanced gold transport and concentration. Similar river drainage reorientation has occurred in other placer fields around the world, but the lack of preserved evidence inhibits documentation of most such changes.

Initial sedimentation and subsequent diagenesis in the Eastern Southland Lignite Basin, southern New Zealand

The Eastern Southland Lignite Basin (ESLB) was initiated in the Late Oligocene at the same time that transpressional deformation began along the Alpine Fault. Initial sedimentation occurred near sea level, with interfingering of marine sediments and non-marine lithic conglomerates and associated lignite measures. Basal conglomerates are immature with rounded and angular clasts, and were locally derived from rising basement rocks on nearby basin margins. Conglomerates in younger lignite measures in the ESLB contain abundant quartz clasts derived from Central Otago schist, 100 km away, as well as some locally derived lithic material. Groundwater movement in the evolving basin caused pervasive clay alteration of lithic conglomerates and the underlying basement down to >10 m below the unconformity, as well as partial alteration of lithic clasts in sandstones. Alteration occurred under chemically reducing conditions and formed kaolinite, ferrous iron-bearing vermiculite-smectite and siderite. Post-depositional uplift in the ESLB has resulted in erosion of more than half of the sediments originally deposited. Erosion of the diagenetically altered rocks rich in clay left only minor residual quartz sand and gravel, with the bulk of the sediment being transported from the basin as clay in suspension or bed-load. In contrast, younger quartz-bearing conglomerates higher in the lignite measures sequence were recycled into the quartz-rich Pliocene conglomerates and younger gravels which now mantle the lignite measures over much of the basin.

Sedimentary documentation of the break-up of Rodinia, Offerdal Nappe, Swedish Caledonides

Late Riphean to early Vendian metasedimentary rocks of the Offerdal Nappe, central Scandinavian Caledonides, were studied by field mapping and measured sections. Even though the rocks have been metamorphosed and tectonically deformed, preserved sedimentary structures and outcrop exposure permit the interpretation of a little-known area with respect to the break-up of Rodinia. This work shows that although the primary sedimentary textures are strongly altered during metamorphism the primary sedimentary structures and bed geometries are commonly preserved. Moreover, the sedimentary structures, where identified, can be used to hydrodynamically estimate the primary grain-size of the metasedimentary rocks. Seven sedimentary facies were identified in three tectonostratigraphic units. The lower tectonostratigraphic unit consists of polymict conglomerates deposited in alluvial fans. The lower part of the middle tectonostratigraphic unit consists of channelled turbidite deposits. The upper part of the middle tectonostratigraphic unit consists of a regressive to transgressive succession with southeast to northwest transition from fluvial deposits, to storm-dominated deposits and into offshore mudstones. The lower part of the upper tectonostratigraphic unit consists of a regressive succession with southeast to northwest transition from channelled turbidites into sheet-like turbidites. The upper part consists of fluvial deposits in updip (southeast) areas, whereas in downdip (northwest) areas the sheet-like turbidites reflect continued deepwater deposition. (1) The ∼1500 m thickness of the Offerdal succession, (2) the occurrence of >300 m thickness of the pebble- to boulder-bearing continental-basement derived alluvial fan conglomerates, (3) the high sediment input rates, and (4) the high subsidence rates suggest an actively subsiding basin and a relatively uplifted hinterland. Furthermore, the arkosic composition of the Offerdal metasandstones, as well as the Precambrian porphyry, syenite and quartzite clast composition of the lithic conglomerates, recognised from the subjacent continental basement of Baltica, suggest a continental sediment source. The rapid accommodation changes, and especially the simultaneous subsidence of downdip areas and uplift of updip areas in UTU2 suggest syn-depositional extensional block rotation. Collectively, this evidence suggests that the Offerdal succession was deposited in a continental rift basin related to break-up of Rodinia.

Vein mineralization at the Damang Gold Mine, Ghana: controls on mineralization

Two distinct styles of Au mineralization occur at the Damang Gold Mine; Palaeoproterozoic sediments of the Tarkwaian Group host both. One style of mineralization is stratabound within quartz–lithic conglomerates of the Banket Series. The second style of mineralization is associated with an extensive low-displacement, fault–fracture mesh that formed in a compressional stress regime late in the deformational history and after the peak metamorphism. Regional deformation within the Tarkwaian involved initial NW–SE directed shortening (D 1). A major NNE-trending F 1 anticline hosts the Damang orebody. Broadly N–S shortening during D 2 resulted in the formation of E–W-trending thrusts with small displacements. The D 3 shortening direction was similar to that of D 1. Steep D 1 faults were reactivated and a new set of low angle thrusts and associated flat-lying extension veins were formed. The bulk of the mineralization observed at Damang is associated with the low displacement D 3 fault fracture mesh. The presence of flat-lying extensional veins and the reactivation of some misoriented D 1 structures is indicative of periodic episodes of supralithostatic fluid pressures, low differential stress and fault-valve behavior towards the end of the deformation history.

The Karoo sediments (Late Palaeozoic to Early Jurassic) of the Ogaden Basin, Ethiopia

The Ogaden Basin is a major sedimentary basin in East Africa located in the south and southeast of Ethiopia, covering an area of about 350,000 km 2. It contains over 5,000 m of Phanerozoic sediments, as evidenced by recent drilling and seismic surveying. A sedimentological investigation has been carried out on cored sections of the Karoo. These are the oldest sediments in the basin, and occur only in the subsurface. Karoo sediments (Permian? to Lower Jurassic) in the Ogaden Basin have an average thickness of about 800 to 1,200 m. The sequence is divided into four formations, from oldest to youngest: the Calub Sandstone, the Bokh Shale, the Gumburo Sandstone and the Adigrat Sandstone. Continental Karoo sedimentation was ended by a progressive Mesozoic marine transgression which initiated the deposition of the overlying Hamanlei Limestone. The basal Calub Sandstone consists of arkosic and lithic conglomerate and sandstone of alluvial fan origin. It is overlain by lacustrine shale, siltstone and minor sandstone of the Bokh Shale. The remaining two formations have a transitional boundary, the Gumburo and Adigrat Sandstones, are fluviatile quartzitic sandstone deposited by braided and meandering rivers, respectively. Initially, Karoo sedimentation took place within an extensional rift, before becoming progressively more widespread during the deposition of the Gumburo and Adigrat Sandstone.

Clarksville Field Red River County, Texas: Production and Facies Interpretation

The Clarksville field was discovered in December in 1985 while targeting a deeper paleozoic horizon. Since production went on line in 1986, this field has produced over 1 million barrels of oil (MMBO) with the appearance of a considerably longer and more lucrative life. The producing horizon is a Jurassic-age lithic conglomerate sitting unconformably on the Paleozoic and Triassic structural front of the buried Ouachita range. Facies correlation out of the basin indicate this unit to be Louark age. Mapping and compositional analysis indicate the depositional environmental of this unit to be an arid climate alluvial fan deposited as a 'Bajada' complex. This fan system was laid down at the updip margin of the actively forming Mesozoic embayment where it meets the Ouachita structural front. The significance of this field is demonstrated by the production yield at a relatively shallow depth (5800 ft). At this time, production similar to Clarksville field has yet to be encountered anywhere along the Mesozoic rim of the East Texas basin but does represent a viable exploration trend, in addition to being a gateway for future paleozoic production in the basin.

Origin of red beds in a Cambrian flysch sequence, Canadian Appalachians, Quebec

ABSTRACTThe basal unit of this flysch sequence consists of about 700 m of rhythmically interlayered red and green claystone intercalated with feldspathic sandstone and lithic conglomerate. Chemical analyses show that both types of claystones have approximately the same Fe2O3 content (8%).The presence within green claystone of inarticulate brachiopods, thin laminations of phosphate and pyrite, coupled with a high Mn content (Fe/Mn : 27) together with the composition of the chlorites, suggest that green mud was deposited slowly in a marine environment under reducing conditions. The hematite content of red claystone averages 4%. The Fe/Mn is high (71), and the clay fraction contains chamosite in addition to chlorite and illite. These characteristics favour a paralic oxidizing environment for the red muds.Rapid resedimentation of red sediment from a paralic environment into the green lithotope, rather than a lateral variation of facies, is suggested by: (1) the distribution of the two hemirhythms in the stratigraphic column, (2) the presence in the red hemirhythm of large slump structures, contorted laminations, and pebbly claystone, and (3) by the internal sedimentary structures of the coarse fraction.

The Mawson Diamictite and the Carapace Sandstone, formations of the Ferrar Group at Allan Hills and Carapace Nunatak, Victoria Land, Antarctica

Abstract Four hundred feet (130 m) plus of current-bedded sandstones and fine lithic conglomerates of Jurassic age at Carapace Nunatak, formerly attributed to the Beacon Group, are shown to consist of re-derived Beacon and primary basaltic volcanic detritus. They are here named the Carapace Sandstone and referred to the Ferrar Group. They are conformably overlain by over 1,000 ft (300 m) of Kirkpatrick Basalt, including a thick hyaloclastite breccia with pillows, and a 50 ft (15 m) intercalation of what is here called Mawson Diamictite. The latter, also Jurassic in age, is also exposed extensively at nearby Allan Hills, where it fills a depression eroded several hundred feet into Permian and Triassic coal measures and sandstones of the Beacon Group. The diamictite consists, like the Carapace Sandstone, of mixed Beacon and basaltic debris, but contains more basalt, is coarser-grained, very poorly sorted, and generally unstratified. More than 1,000 ft (300 m) thick, it was originally called Mawson Tillite. ...