Arkosic Rocks Research Articles

Provenance of Pennsylvanian–Permian sedimentary rocks associated with the Ancestral Rocky Mountains orogeny in southwestern Laurentia: Implications for continental-scale Laurentian sediment transport systems

AbstractThe Ancestral Rocky Mountains system consists of a series of basement-cored uplifts and associated sedimentary basins that formed in southwestern Laurentia during Early Pennsylvanian–middle Permian time. This system was originally recognized by aprons of coarse, arkosic sandstone and conglomerate within the Paradox, Eagle, and Denver Basins, which surround the Front Range and Uncompahgre basement uplifts. However, substantial portions of Ancestral Rocky Mountain–adjacent basins are filled with carbonate or fine-grained quartzose material that is distinct from proximal arkosic rocks, and detrital zircon data from basins adjacent to the Ancestral Rocky Mountains have been interpreted to indicate that a substantial proportion of their clastic sediment was sourced from the Appalachian and/or Arctic orogenic belts and transported over long distances across Laurentia into Ancestral Rocky Mountain basins. In this study, we present new U-Pb detrital zircon data from 72 samples from strata within the Denver Basin, Eagle Basin, Paradox Basin, northern Arizona shelf, Pedregosa Basin, and Keeler–Lone Pine Basin spanning ∼50 m.y. and compare these to published data from 241 samples from across Laurentia. Traditional visual comparison and inverse modeling methods map sediment transport pathways within the Ancestral Rocky Mountains system and indicate that proximal basins were filled with detritus eroded from nearby basement uplifts, whereas distal portions of these basins were filled with a mix of local sediment and sediment derived from marginal Laurentian sources including the Arctic Ellesmerian orogen and possibly the northern Appalachian orogen. This sediment was transported to southwestern Laurentia via a ca. 2,000-km-long longshore and aeolian system analogous to the modern Namibian coast. Deformation of the Ancestral Rocky Mountains slowed in Permian time, reducing basinal accommodation and allowing marginal clastic sources to overwhelm the system.

Arkosic rocks from the San Andreas Fault Observatory at Depth (SAFOD) borehole, central California: Implications for the structure and tectonics of the San Andreas fault zone

The San Andreas Fault Observatory at Depth (SAFOD) drill hole encountered indurated, high-seismic-velocity arkosic sedimentary rocks west of the active trace of the San Andreas fault in central California. The arkosic rocks are juxtaposed against granitic rocks of the Salinian block to the southwest and against fi ne-grained Great Valley Group and Jurassic Franciscan rocks to the northeast. We identify three distinct lithologic units using cuttings, core petrography, electrical resistivity image logs, zircon fi ssion-track analyses, and borehole-based geophysical logs. The upper arkose occurs from 1920 to 2530 m measured depth (mmd) in the borehole and is composed of fistructural blocks defi ned by bedding orientations, wireline log character, physical properties, and lithologic characteristics. A clay-rich zone between 2530 and 2680 mmd is characterized by low V p and an enlarged borehole. The lower arkose lies between 2680 and 3150 mmd. Fission-track detrital zircon cooling ages are between 64 and 70 Ma, appear to belong to a single population, and indicate a latest Cretaceous to Paleogene maximum depositional age. We interpret these Paleocene‐Eocene strata to have been deposited in a proximal submarine fan setting shed from a Salinian source block, and they correlate with units to the southeast, along the western and southern edge of the San Joaquin Basin, and with arkosic conglomerates to the northwest. The arkosic section constitutes a deformed fault-bounded block between the modern strand of the San Andreas fault to the northeast and the Buzzard Canyon fault to the southwest. Signifi cant amounts of slip appear to have been accommodated on both strands of the fault at this latitude.

An amagmatic origin of carlin-type gold deposits

Carlin-type deposits are major sources of gold, yet their origins are enigmatic. Suggested genetic models make connections to magmatism, regional metamorphism, or regional extension. Depositional mechanisms are uncertain as well. We propose on the basis of geologic, physical, and chemical reasoning, a genetic model in which meteoric fluids were circulated by heat released during crustal extension. These fluids interacted at depth with the sedimentary rock pile and scavenged gold. Upon upwelling, these fluids interacted with various lithologies and/or other fluids and produced the characteristic alteration and metal suites of these deposits. To test the viability of this amagmatic model, we have investigated certain physical and chemical constraints implicit to the model. Heat balance calculations indicate that ample surface waters could be heated to appropriate temperatures and circulated during rapid crustal extension. Mass balance calculations indicate that solution transport efficiencies of <10 percent can account for several times the amount of known alteration and mineralization. In contrast, magma-driven or metamorphic fluids appear to be less adequate sources. A two-step, source to deposit approach was used to simulate fluid-rock interaction and mass transport implied in the amagmatic model. Fluids with a fixed Sigma Cl were first equilibrated with various potential source rocks (arkose, graywacke, and pelite, each with various f O2 and f S2 buffer assemblages) at 300 degrees C and 1 kbar to simulate reaction at depth, perhaps near the brittle-ductile transition. These fluids were then reequilibrated under conditions indicated for deposit formation (225 degrees -150 degrees C, 0.5 kbars) to investigate processes including cooling, wall-rock reaction, and mixing which could be important to ore deposition. Gold transport was favored for low-chlorinity, intermediate oxidation state (below pyrrhotite-pyrite-magnetite), slightly acid fluids from sulfide-bearing sources (i.e., equilibration with arkosic rocks). These conditions are not conducive to base metal transport and explain the rarity of base metals in Carlin-type systems. When reacted under deposit level conditions, these calculated ore fluids leach carbonate and deposit quartz. The volume ratio of carbonate dissolved to quartz precipitated is approximately 0.2 to 2.5, and increases when mixing with a second fluid is considered. Arkose-equilibrated source fluids can precipitate gold by combinations of wall-rock reaction with a decarbonated host, mixing with various sulfide-poor fluids, and reaction with iron-rich host rocks. Cooling alone is ineffective over this temperature interval. Although most deposits are hosted by decarbonated marls, geochemical calculations suggest that other depositional environments are feasible, consistent with observation. Although here applied to the amagmatic hypothesis, these chemical calculations are not mechanism specific and might also be appropriate to metamorphic or magma-driven hydrothermal systems. These results demonstrate that Carlin-type deposits can result from the coincidence of an effective driving force (rapid, major crustal extension), a source of dilute fluids, and chemically appropriate sets of both source and host rocks. The aqueous chemistry involved is unexceptional and shares many features with other types of moderate- to low-temperature gold-bearing hydrothermal systems--it is the geologic setting, and perhaps the fluid driving force, that is unusual. An amagmatic model is consistent with both deposit-scale and regional-scale observations; it rationalizes Carlin-type systems in the framework of southwestern North American metallogeny and it implies multiple possible mechanisms for gold mineralization. Detachment-type gold systems may be the high-oxidation state, high-salinity analogues of Carlin-type systems, formed by major extension, but from contrasting lithologic and fluid sources.

Geochemistry of high-grade supracrustal rocks from the Kerala Khondalite Belt and adjacent massif charnockites, South India

The Kerala Khondalite Belt (KKB) comprises a suite of supracrustal rocks of probable Proterozoic age that was metamorphosed to the granulite facies. The KKB consists of an interlayered sequence of: (1) garnet-biotite ± orthopyroxene ± graphite gneisses; (2) khondalites (graphite-garnet-biotite-sillimanite ± cordierite gneisses); and (3) cordierite gneisses (garnet-biotite-cordierite ± orthopyroxene), as well as less abundant mafic granulites, calc-silicates and quartzites. Petrologic and geochemical data suggest that these rocks were originally arkosic and argillaceous sediments (possibly accompanied by felsic volcanic rocks), along with minor intercalations of basalts, carbonates and quartz-rich sandstones. The REE patterns of the metasediments are typically LREE enriched with large negative Eu anomalies, indicating a continental provenance. The major- and trace-element compositions of some of the arkosic rocks of the KKB resemble those of the massif charnockites that bound the KKB to the north which suggests that the massif charnockites may have been the source of some of the KKB sediments. Modern-day analogues of the pelite-arkose lithologic association observed in the KKB occur in intracratonic rifts and rifted continental margins. This association is clearly distinct from the graywacke-mafic volcanic association found in the southern Indian greenstone belts and many other lower-grade Archean sedimentary sequences. The KKB supracrustals show both lithologic and chemical similarities to many Proterozoic supracrustal successions.

Geologic setting and tertiary structural evolution of southwestern Arizona and southeastern California

Volcanic and sedimentary rocks and structures record the Tertiary structural evolution of the lower Colorado River region in southwestern Arizona and southeastern California. A late Eocene or early Oligocene (prior to ∼33 Ma) episode of faulting is indicated by medium‐ to coarse‐grained arkosic rocks in the Chocolate and southern Trigo Mountains. Much of the area was relatively quiescent tectonically during extrusion of volcanic rocks from ∼33 to 22 Ma, but the southernmost part was periodically uplifted and eroded into its underlying crystalline rocks. A major episode of extensional deformation and tilting occurred after deposition of welded tuff about 22 Ma and affected the entire area from the Palo Verde Mountains on the west to the Kofa Mountains on the east. Extension‐related faulting quickly waned and had largely ceased by about 20 Ma in the Kofa Mountains; elsewhere the timing is poorly constrained. By ∼13 Ma, thick alluvial fans filled many grabens and half grabens among tilted fault blocks throughout the area. Volcanism in the lower Colorado River region may have been coincident with a broad structural depression now oriented east‐west. The northern limit of the volcanic terrane defines a tilt‐domain boundary. The northern boundary, reaching from the New Water Mountains in Arizona to the little Chuckwalla Mountains in California, ultimately evolved to separate a terrane of relatively untilted crystalline horsts on the north from a series of east or northeast dipping fault blocks on the south. The southern margin is less well defined but is subparallel to the northern boundary and to the Chocolate Mountains anticlinorium.

Origen de arcillas alumínicas en horizontes de alteración de materiales graníticos del borde sur de la sierra de Guadarrama

Three weathering profiles of granitic rocks and two arkoses in the NW of Madrid Province have been studied. Rocks are granodiorites and adamellites with albites showing variable degrees of sericitization, and no-weathered potassic feldspars. Clay fraction in the weathered horizons from these rocks displays high content in beidellite, and minor amounts of illite and 7 Ǻ-halloysite. On the other hand, beidellite, illite and well crystallized kaolinite are clay minerals in the arkosic rocks. Microfabric characteristics from SEM studies show abundant weathered albites with clean surfaces and formation of beidellite in all areas of the samples. Moreover, spatial relations between albite and beidellite are not observed. In pedochemical weathering of albite, sodium from this mineral should be exchanged with protons from soil solution with very quick kinetics. This initial alkalinization produces an increase in the silica solubility released from albite network. In these environments, a continued contact between weathering solution and albite causes a silica and alumina enrichment and beidellite or 7 Ǻ-halloysite precipitation. Microclimatic variations in weathering environment may cause fluctuations in silica and alumina activities with resultant pH changes. So, when silica activities are high, beidellite neoformation is favoured; on the contrary, 7 Ǻ-halloysite is precipitated.

Source terranes for Proterozoic sedimentary rocks in southern British Columbia: Nd isotopic and petrographic evidence

Quartz arenites of the Fort Steele Formation, the lowest exposed stratigraphic unit of the Purcell Supergroup, have Nd crustal residence times (Tcr) of 2.61–2.63 Ga. These overlap the low end of the Tcr range of the polymetamorphic Kenoran–Hudsonian basement of southern Alberta and southwestern Saskatchewan. The Tcr values of overlying fine-grained Purcell rocks average 2.0 Ga.Near Lake Windermere, the basal Toby conglomerates of the Windermere Supergroup, apparently derived from underlying Purcell strata, have Tcr values of 1.98 and 2.03 Ga. Stratigraphically higher argillite and arkoses range from 2.35 to 2.54 Ga. This higher range indicates contributions from a source terrane different from the main body of the Purcell Supergroup.Our interpretation of events in the East Kootenay area is that early Purcell sedimentation (Fort Steele Formation) was derived from reworking of the crystalline basement of the Alberta shelf as the Beltian Sea transgressed. As the basin deepened, it was filled by fine-grained turbidites from a distal southern or southeastern source. Later Purcell strata have more complex paleocurrent direction patterns but relatively uniform Tcr values similar to those of the earlier turbidites. Following the East Kootenay orogeny, the basal conglomerate of the Windermere Supergroup (Toby Formation) consisted dominantly of locally derived clasts. These Purcell-derived strata grade upward into arkosic rocks with an admixture of Archean detrital material, probably from a proximal eastern source.

Provenance of Conglomerate Clasts from Upper Cretaceous Kuskokwim Group, Southwest Alaska: ABSTRACT

The predominantly Upper Cretaceous (Albian to Coniacian) Kuskokwim Group consists of marine turbidites and subordinate fluvial and shallow marine strata, deposited in an elongate southwest-trending basin covering over 70,000 km2 in southwestern Alaska. In the Sparrevohn and Cairn Mountain areas of the Lime Hills A-7 and A-8 quadrangles, fluvial, inner fan, middle fan, and outer fan facies are stacked with distal facies over proximal facies by northwest vergent thrust faults. Inner fan pebble-to-cobble conglomerate and pebbly sandstone were deposited as submarine grain flows up to 10 m thick containing reversely graded bases and normally graded tops. Clasts from these conglomeratic deposits are predominantly sedimentary rock fragments--particularly sandstone, si tstone, and argillite--originally thought to have been derived from the nearby Jurassic and Lower Cretaceous flysch (Kihiltna terrane). Detailed examination of these clasts, however, indicate that they contain as minor constituents Paleozoic coral, oolitic limestone, algal boundstone, radiolarian chert, and mafic, intermediate, and felsic volcanic rocks, most likely derived from the adjacent Nixon Fork, Dillinger, and Mystic terranes. Sandstone clasts are arkosic (Q25-F37-L38, n = 7) and contain subequal amounts of K-feldspar and plagioclase. Sand grains within these clasts are moderately sorted, subrounded, and have presolved contacts. Similar arkosic rocks have been described from the Dillinger terrane of the McGrath quadrangle and are the most likely source for the pebbles and cobbles within the Kuskokwim Group. End_of_Article - Last_Page 660------------

Evidence for the influence of fCH 4 on the crystallinity of disseminated carbon in greenschist facies rocks, Rhode Island, USA

Disseminated carbon has been extracted from 19 samples of arkosic rocks from the chlorite, biotite and garnet zones in the Narragansett basin, and analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD of these samples yields only the (002) peak which may be as much as four times wider than the silicon internal standard peak and is always skewed to low angles 2θ. All (002) peaks were digitized and the four moments calculated. The results suggest that width at half-height (W1/2) provides a poorer estimate of peak shape than coefficients of skewness and kurtosis. The positive correlation of these coefficients with grain size determined by SEM in the range of 0.1–100 μm suggest a direct relationship between peak shape and grain size.

Shallow-Water Upper Jurassic Rocks Dredged From Bering Sea Continental Margin: ABSTRACT

Rocks recently dredged along the Bering Sea continental slope include fossiliferous, gray-green arkosic sandstone of Late Jurassic age. The sandstone was recovered from acoustic basement at water depths ranging from 1,500 to 2,800 m. These arkosic rocks were sampled at nine sites along a segment of the Beringian margin that extends about 550 km northwest of the Pribilof Islands toward eastern Siberia. Preliminary lithologic and petrographic examination of the feldspathic sandstone and lesser amounts of siltstone indicates that these rocks are equivalent to units in the Naknek Formation, End_Page 490------------------------------ which is exposed in southern Alaska and along the Alaska Peninsula. The megafossil Buchia rugosa was found in quantity in one dredge haul. This Late Jurassic pelecypod implies that the rocks were deposited in a neritic or shallow-water environment. These Jurassic strata are overlain unconformably by diatomaceous mudstone or sandstone as old as late Eocene or early Oligocene. Geophysical work indicates that the Jurassic rocks were recovered from an acoustic basement complex that can be traced northwestward from near the western tip of the Alaska Peninsula to Siberia, a distance of nearly 1,250 km. The Mesozoic basement complex consists structurally of a series of interconnected ridges that underlie the outer shelf and crop out along the adjacent continental slope. Previous theories on the tectonic evolution of the Bering Sea implied that the continental margin should be underlain either by (1) deformed Mesozoic trench or slope deposits that were structurally accreted to the margin by oblique convergence between the Kula(?) and North American plates or (2) by disrupted fragments of Mesozoic slope beds deposited along a transform or strike-slip boundary that separated the two plates. However, rocks dredged from the margin now indicate that a belt of shallow-water Upper Jurassic sandstone underlies the Beringian margin between southwestern Alaska and eastern Siberia. This belt, which structurally may include younger rocks, subsided in early Tertiary time to form the existing Beringian margin. Collapse along the margin was more than 3 or 4 km; in some areas beneath the outer shelf, the Mesozoic framework may have subsided more than 10 km. End_of_Article - Last_Page 491------------

Crustal and upper mantle stratigraphy beneath eastern Kansas

The kimberlites of Riley County, Kansas contain xenoliths of upper mantle and crustal rocks. The kimberlite probably originated below the Low Velocity Zone (LVZ). Deep seated xenoliths include ilmenite‐pyroxene symplectites, lherzolite, and eclogite. Other xenoliths from below the Moho include pyroxenites, plagioclase‐bearing eclogites, and several types of granulites. Above (or near) the Moho (35 kilometers depth) occur granulites overlain by metanorites which in turn are overlain by noritic‐gabbroic rocks and diorites. Metamorphic amphibolite and schistose xenoliths are rare, as too are granitic rocks. Presumably the granites occur below the Rice Formation (Arkosic Rocks) and Clay Formation (Basaltic Rocks) which are in fault contact with each other and extend into the midcontinental region from the Lake Superior Province. Above the Precambrian basement, there occurs approximately 1.7 kilometers of Paleozoic shales and limestones; xenoliths of these rocks are common in the kimberlite diatremes.

Discussion

Dr H. G. Reading queried the author's interpretation of the Moy Sandstone as entirely marine. The unimodal palaeocurrent directions pointing towards the basin centre, the feldspathic nature of the grits and the local derivation of the clasts would all seem to point towards a fluvial rather than a marine origin. Was there any positive evidence for a marine origin, such as polymodal palaeocur-rents, wave ripples, tidal currents, well sorted compositionally mature beach or bar sandstones ? The calcareous nature of the cement was no reason for postulating a marine origin because many fluvial sandstones have a calcite cement. Could this sequence of sandstones represent environmental changes from fluvial below into shallow marine above? If this was found to be so, it would suggest a southerly source area of much wider extent than the relatively small islands postulated by Dr Dixon. In a written reply, the A uthor said, any impression conveyed that the Moy Sandstone had a history which was entirely marine was quite unintentional. Indeed the account of the history of sedimentation mentioned the accumulation of both fluvial and marginal marine detrital sediments during the Visèan transgression, although evidently it failed to give due emphasis to the initial fluvial aspects of the sequence. The isolated exposures along the headwaters of the River Moy show a pronounced change from dominantly cross-bedded, immature, coarse-grained, arkosic rocks in the lower part to planar bedded, near-mature, finer grained rocks in the upper part. This is in keeping with a sequence of environmental changes from

The origin of the southern Namaqualand gneiss complex, South Africa, in the light of geochemical data

The Namaqualand granite-gneiss complex in South Africa covers an area of approximately 70,000 km 2. The southern portion consists of mainly mesozonal biotite gneiss and a fine-grained granulite. Geochemical data suggest that regional metamorphism and possibly partial mobilisation of mainly arkosic rocks of the Kheis System gave rise to the basement gneiss in which remnants of the transformed sediments are still preserved. There is no evidence for large-scale metasomatic granitisation which, according to previous authors, transformed the original sediments into the gneisses. Petrological considerations strongly support the conclusion that the sediments of the previously called ‘Malmesbury Formation’ cannot be the parent rocks of the gneiss complex.