Christopher A. Suczek

Plate Tectonics and Sandstone Compositions

Detrital framework modes of sandstone suites from different kinds of basins are a function of provenance types governed by plate tectonics. Quartzose sands from continental cratons are widespread within interior basins, platform successions, miogeoclinal wedges, and opening ocean basins. Arkosic sands from uplifted basement blocks are present locally in rift troughs and in wrench basins related to transform ruptures. Volcaniclastic lithic sands and more complex volcano-plutonic sands derived from magmatic arcs are present in trenches, forearc basins, and marginal seas. Recycled orogenic sands, rich in quartz or chert plus other lithic fragments and derived from subduction complexes, collision orogens, and foreland uplifts, are present in closing ocean basins, diverse succ ssor basins, and foreland basins. Triangular diagrams showing framework proportions of quartz, the two feldspars, polycrystalline quartzose lithics, and unstable lithics of volcanic and sedimentary parentage successfully distinguish the key provenance types. Relations between provenance and basin are important for hydrocarbon exploration because sand frameworks of contrasting detrital compositions respond differently to diagenesis, and thus display different trends of porosity reduction with depth of burial.

Provenance of Franciscan graywackes in coastal California

A systematic comparison of available detrital modes for graywacke sandstones of the Franciscan subduction complex and for coeval sandstones of the Great Valley sequence in the California Coast Ranges indicates that both were apparently derived from the same general sources. The inferred provenance terrane was the ancestral Sierran-Klamath magmatic arc, from which mixed volcanic and plutonic detritus readily entered the adjacent Great Valley forearc basin. At intervals along the trend of the arc-trench system, arc-derived detritus also bypassed the forearc region through submarine canyons that fed the Franciscan trench. Longitudinal flow along both the Great Valley trough and the Franciscan trench achieved wide dispersal of the turbidite sediment. Suites of both Franciscan and Great Valley samples include an array of subquartzose compositions ranging from feldspatholithic to lithofeldspathic. Mean framework modes of 17 Franciscan suites comprising 203 individual samples, and of 23 Great Valley suites comprising 410 individual samples, range from 14% to 44% quartz grains, 15% to 54% feldspar grains, and 7% to 71% total lithic fragments. The ratio of quartz to feldspar remains relatively constant as the proportion of lithic fragments changes. The compositional variations reflect differences mainly in the admixture of lithic fragments derived principally from volcanic cover with quartz, and feldspar derived principally from erosion of underlying plutons. Despite major overlap in the compositions of the two sets of samples, some Franciscan sandstones are somewhat more feldspathic and less lithic than any known Great Valley counterparts and were probably derived from segments of the arc terrane where exposures of plutons were more extensive than within typical Great Valley sources. Higher proportions of non-volcanic to volcanic lithic fragments in some Franciscan sandstones probably reflect complex recycling processes on the trench slope. Diagenetic effects in many Franciscan suites include apparent wholesale replacement of K-feldspar by albite. Present age control is inadequate to test fully for time-dependent trends in the compositions of Franciscan sandstones analogous to the known stratigraphic variations in the composition of Great Valley sandstones. This question ought to be investigated in future studies.

Petrology and Provenance of Cenozoic Sand from the Indus Cone and the Arabian Basin, DSDP Sites 221, 222, and 224

ABSTRACT The closing of the remnant ocean basin between India and Asia has produced the uplifted gneissic, sedimentary, and metasedimentary terrane of the western Himalayas. Sediment derived from this source is presently being deposited in the submarmefan and basin-plain complex of the Indus Cone and Arabian Abyssal Plain, the location of DSDP sites 221,222, and 224. Point-counts of 15 Cenozoic sand samples from these sites have mean QFL percentages of 43-30-27. The plagioclase/total-feldspar mean is 0.66. Lithic types are dominated by quartz-mica tectonite, argillite-shale, polycrystalline carbonate, and other sedimentary and metasedimentary varieties. Volcanic grains present are mainly felsic. These sands resemble those of the Bengal and Nicobar Fans, which were also derived from the Himalayas. These plus other sands derived from collision orogenic belts have a distinctive signature: P/F close to 0.66; Lv/L low, 0.10 or less; and Q dominant, with F and L subequal.

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Sandstone Compositions Related to Plate Tectonic Settings: ABSTRACT

Sandstone compositions are a function of provenance and depositional basin and both of these are determined by plate tectonics. Modal analyses of major framework grain types--quartz, polycrystalline quartz, potassium feldspar, plagioclase, volcanic lithic grains, and sedimentary lithic grains--plotted on a series of four triangular diagrams can be used to distinguish between the main provenance types. Quartz-rich sands come from cratonal sources and are deposited in basins on the craton and at quiet continental margins (miogeoclines and opening ocean basins). Arkosic sands are shed from uplifted blocks on continental basement into rift troughs and wrench basins associated with transform faults. Volcanic lithic sands have volcanic arc provenances and are deposited in trenches, forearc basins, and marginal seas. Undissected arcs produce very lithic-rich sand; more mature and eroded arcs produce a mixture of volcanic lithic and plutonic (mainly quartz and feldspar) detritus. Sands rich in quartz or chert plus sedimentary lithic grains come from subduction complexes, collision orogenic belts, and foreland uplifts and are deposited in closing ocean basins, successor basins, and foreland basins. Data from both modern sands and ancient sandstones of known tectonic settings fit the above picture; influence of climate and diagenesis on sand composition must be less important than that of tectonic setting. Hence knowing the detrital modes of sandstones provides a way of determining the original tectonic setting of the rocks, and framework grain composition of sands can be predicted from their tectonic setting. End_of_Article - Last_Page 449------------