Detrital Modes Research Articles

Provenance of Impure Calclithites in the Laramide Foreland of Southwestern Montana

ABSTRACT The Sphinx Conglomerate is an uppermost Cretaceous and Paleocene(?) synorogenic clastic wedge derived from a Laramide uplift in southwestern Montana. Conglomerate petrology documents unroofing of the thick Phanerozoic section that covered the uplift. The active tectonic setting resulted in close correspondence between source lithologies and derived conglomerates and sandstones. Sandstone detrital modes determined using the Gazzi-Dickinson point-counting method further document the unroofing sequence. All Sphinx sandstones consist of mixtures of siliciclastic and carbonatoclastic detritus. Detrital modes correspond to the recycled orogenic provenance of other workers. In addition to these general modal compositions, vertical trends in QFL%Q and QFL%L, and in proportions of types of sedimentary lithic fragments mimic trends in the inverted Phanerozoic stratigraphic section. Total quartz (Q) decreases upsection in the Sphinx, corresponding to decreases of both sandstone (the only source for Qm) and chert (source for Qp) in the inverted Phanerozoic section. Upward increases occur in unstable lithic fragments in the Sphinx, and in carbonate units in the inverted Phanerozoic section. Carbonate sedimentary lithic fragments (Lsc) increase vertically in proportion to silicic sedimentary lithic fragments (Lss) and chert (Qp) fragments, again mimicking the inverted Phanerozoic section. Detailed modal analysis of recycled orogenic sandstones should include study of subcategories of sedimentary lithic fragments, for example, QpLssLsc and QLssLsc plots. Standard QFL and QpLvmLsm plots are useful in assessing the overall tectonic setting of foreland sandstones but are insensitive to temporal changes in source areas. Supracrustal sources contribute most or all of the detritus to foreland sandstones. Therefore, proportions of sedimentary lithic fragments record tectonic and erosional processes in source areas through time.

Petrology and Provenance of Modern Sands from Cascade Range Forearc and Canadian Rocky Mountain Fold-Thrust Belt: ABSTRACT

The Cascade Range volcanic arc and forearc, and the Canadian Rocky Mountain fold-thrust belt represent the two sides of a continental margin arc-trench system. Sands from these areas show clear compositional differences. The most significant discriminating parameters are volcanic lithic grains, metamorphic lithic grains, plagioclase-to-feldspar ratio, and quartz. Variable sediment composition is also evident within each setting. Cascade sands are volcaniclastic and have high plagioclase-to-feldspar ratios. They divide into three categories (volcanic arc, alluvial forearc, and coastal forearc) that differ in their lithic contents and plagioclase-to-feldspar ratios. These changes reflect the attrition of volcanic lithics with distance from the arc and the input of recycled sediment and subduction-complex lithologies. Rocky Mountain sands are sedimenticlastic. They are of two types, a miogeocline-shelf provenance and a clastic-wedge provenance. These linear belts differ in clastic-carbonate content, plagioclase-to-feldspar ratio, and quartz content. The compositional differences reflect interstratified petrofacies of fold-thrust belts. Just as they can use detrital modes of modern sands to characterize provenance and tectonic setting, modes of ancient sandstones help up to recognize provenance terranes and reconstruct paleotectonic settings.

Sediment provenance within subduction complexes—An example from the Aleutian forearc

The ultimate composition of any sandstone is affected by a host of primary and secondary factors, including the lithologies present in source terranes, climate, depositional environment and diagenesis. In the case of a subduction complex, however, unequivocal identification of detrital provenance may be impossible because of the cumulative effects of tectonic and sedimentary transport. Long-distance sedimentary transport (> 1000 km) is common within trenches, and abyssal-plain turbidites can be tectonically transported for long distances as the underlying oceanic basement drifts towards a subduction front. Post-accretionary displacement can occur as a consequence of strike-slip faulting, and the total distance of tectonic dislocation may reach several thousand kilometers. The present-day Aleutian forearc region (North Pacific Ocean) illustrates many of the “problems” which typify subduction zones. Several petrologic suites can be identified, and there are significant variations in detrital modes in both time and space. The Aleutian region serves as a sobering modern analog for accreted rock units such as the Franciscan Complex of California, where intercalations of discrete sandstone suites have been noted. In the absence of paleomagnetic control, interpretations of sediment provenance within ancient subduction complexes probably should be restricted to the generic level.

Geologic Description of Upper Miocene Chanac and Lower Pliocene Etchegoin Formations, Part 1: Depositional Environments, Reservoir Characteristics, and Petrology, Kern Front Field, California: ABSTRACT

Twenty-four cores (221 ft) of the upper Miocene Chanac and lower Pliocene Etchegoin Formations from three wells in Kern Front field, California, were analyzed to determine their depositional environments and reservoir characteristics. The Etchegoin Formation is interpreted to be a progradational shoreface deposit consisting of a basal transgressive unit overlain by multistacked, storm or tidally influenced, foreshore sand ridges attached to the shoreline. These shoreface deposits are inferred to trend north-south, and are interbedded with and overlain by bioturbated units and marine channels. The Etchegoin is stratigraphically between the younger nonmarine Kern River and older Chanac Formations, both meandering stream units deposited on a relatively low-relief, narrow coastal plain. East-west-trending channel, interchannel, and sheetwash facies are recognized in the Chanac Formation. Log-derived data combined with porosity and permeability measurements indicate that these Miocene and Pliocene sandstones have excellent porosity (average 35%) and permeability (up to 8.5 darcys). The sandstones are poorly indurated, arkosic arenites and wackes composed of subangular, medium to coarse-grained, and poorly sorted detritus. Detrital modes for the sandstones are Q/sub 33/ F/sub 47/ L/sub 20/ and Q/sub 38/ P/sub 48/ K/sub 14/. Authigenic minerals are rare and include calcite, illite, calcium zeolite, and collophane. The best reservoirmore » sandstones in the field are the Etchegoin marine foreshore and Chanac channel facies. The three main factors controlling reservoir quality appear to be: (1) the percentage of fines ranging from 11-12% in good reservoir facies to 22-45% in poorer quality reservoir facies; (2) the percentage of gravel or coarser grained fraction (sand or gravel), which ranges from 83 to 54%; and (3) sorting.« less

Source Rock Versus Sedimentary Control on The Mineralogy of Deltaic Volcanic Arenites (Upper Triassic, Northern Italy)

ABSTRACT The Carnian sandstones of the Southern Alps are fluviodeltaic and lagoonal clastic wedges derived from volcanic source rocks changing with time from mesosilicic to persilicic lavas and pyroclastics. Two-thirds of the variance in detrital modes reflect this petrologic evolution of the source. The effect of sedimentary processes is never strong enough to obscure the fundamental relationship of sandstone composition to provenance, but it accounts for most short-term variability in quartz/feldspar and feldspar/ volcanic rock fragment ratios. Plagioclase and quartz grains are concentrated, respectively, in the very fine to fine fractions and in medium-grained sandstones. The quartz/feldspar ratio (Q/F) thus decreases through delta-plain upward-fining sequences, from channel-lag sediments ( /F = 1/3) to overbank sands (Q/F = 1/8). The sedimentary control on the composition of fluviodeltaic sands results from the coupling of mechanical effects in high-gradient headwater streams and hydraulic sorting in low-gradient depositional environments. Assuming that the effectiveness of mineralogical segregation in different grain-size fractions during transport depends primarily on paleorelief, a decrease in altitude during Carnian times is inferred within the Triassic volcanic source, which lay south of the sedimentary depocenter. Reworking by weak traction currents in lagoonal settings has further increased the textural maturity and the mineralogical stability of the sands. Volcanic rock fragments were reduced by selective destruction in marine environments, but the Q/F ratio was no affected, and a common value of 1/6 characterizes all lagoonal strata.

Sandstone Detrital Modes, Central Utah Foreland Region: Stratigraphic Record of Cretaceous-Paleogene Tectonic Evolution

ABSTRACT Within a sedimentary basin, the analysis of sandstone petrofacies defined by detrital modes of framework grains is a powerful means to detect areal or temporal changes in provenance related to evolving paleotectonics and paleogeography. In the central Utah foreland, the regional tectonic transition from thin-skinned Sevier thrusting during the Late Cretaceous to deep-seated Laramide deformation during the Paleogene is recorded by four contrasting petrofacies in uppermost Cretaceous and Paleogene sandstones of fluvial and deltaic facies. Coeval subaqueous environments were marine during the Cretaceous and lacustrine during the Paleogene. The Cretaceous quartzose petrofacies (Qm93, F1, Lt6) of mainly Campanian age was derived from sedime tary sources in the active Sevier thrust belt, and transported into a broad adjacent foreland basin by transverse paleocurrents. The heterogeneous Cretaceous quartzolithic petrofacies (Qm40-70, F5-20, Lt25-40) of uppermost Campanian age was derived from the Sevier orogenic belt during the waning phases of Sevier tectonism, and was distributed by longitudinal paleocurrents within a foreland basin whose floor had begun to be deformed by incipient Laramide tectonism. The Paleogene feldspathic petrofacies (Qm50, F35, Lt15) in Paleocene and Eocene strata is composed of arkosic detritus that was derived principally from basement rocks then exposed in the extensive San Luis uplift of Laramide age in south-central Colorado. The feldspathic sand was transported to the Laramide Uinta Basin and vicinity in Utah by a trunk river which flowed generally northwestward between monoclinal Laramide uplifts of the Colorado Plateau. The Paleogene quartzolithic petrofacies (Qm75, F2, Lt23) of dominantly Eocene age was derived mainly from sedimentary sources in the Laramide Uinta uplift to the north, but also in part from similar sources in the dormant Sevier thrust belt to the west. In the Rocky Mountain and intermountain regions, more widespread and systematic use of petrofacies analysis, in combination with standard paleocurrent and lithofacies analyses, could lead to better definition of the timing of Laramide uplift and improved understandi g of the evolution of complex Laramide paleogeography.

Petrology of the Archean Pontiac and Kewagama sediments and implications for the stratigraphy of the southern Abitibi belt: Discussion

Lajoie and Ludden presented data on the petrography, traceelement chemistry, and bedding characteristics of the Pontiac and Kewagama sedimentary units from which they proposed a paleogeographic environment of sedimentation and discussed the relative position of these units in the stratigraphy. Most of the sedimentological and chemical data concord with our paleogeographical model (Dirnroth et al. 1982) and as such they are a valuable addition to our paper. Thus, textural parameters, sedimentary structures, and bedding characteristics suggest that Pontiac sediments are turbidites derived from the subaerial erosion of a northern high-relief and tectonically unstable source area and that they accumulated in a relatively deep-water environment, below wave base (Lajoie and Ludden, p. 1310). Trace-element compositions of the two sedimentary units indicate sources that are partly volcanicplutonic in nature and fractionated in lightlheavy rare-earth elements (REE) relative to the adjacent Blake River Group (Lajoie and Ludden, p. 13 11). ~ h e s e chemical data are consiitent with the REE contents of three mica schists of the Pontiac Group published by Ujike (1984). The petrography of the Kewagama sediments also suggests a derivation from a nearby subaerial source of high relief and mostly volcanic composition (Lajoie and Ludden, p. 131 1). Unfortunately, Lajoie and Ludden's interpretation of the paleogeographic model is weakened by a misrepresentation of the Pontiac metasediments as being metaquartzite with 'framework' consisting of more than 90% quartz. This statement is based on an evaluation of the detrital mode of the composition of Pontiac metasediments as presented in the Qm-P-L, triangle of Dickinson and Suczek (1982) (Lajoie and Ludden, Fig. 3). However, it is inappropriate to evaluate the framework composition of strongly deformed and metamorphosed greywackes in the Q,-P-L, triangle as Lajoie and Ludden have done. Ingersoll and Suczec (1984, p. 985) warn:

The effect of grain size on detrital modes; a test of the Gazzi-Dickinson point-counting method; discussion and reply

The effect of grain size on detrital modes; a test of the Gazzi-Dickinson point-counting method; discussion and reply

The effect of grain size on detrital modes; a test of the Gazzi-Dickinson point-counting method; discussion and reply

The effect of grain size on detrital modes; a test of the Gazzi-Dickinson point-counting method; discussion and reply

The Effect of Grain Size on Detrital Modes: A Test of The Gazzi-Dickinson Point-Counting Method: REPLY

The Effect of Grain Size on Detrital Modes: A Test of The Gazzi-Dickinson Point-Counting Method: REPLY

Provenance of Selected Sandstones and Mud Rocks of Dunkard Group (Upper Pennsylvanian-Permian) in Ohio, West Virginia, and Pennsylvania: ABSTRACT

A standard QFL count and a special count of quartz grains indicate that the Dunkard sandstones are rich in quartz and sedimentary and metasedimentary lithic fragments, but poor in feldspar and igneous lithic fragments. Plots of the Dunkard detrital mode on provenance diagrams End_Page 1445------------------------------ reveal a separation of samples into two groups: a geographically restricted group and a more extensive southern group. This discrimination is consistent with paleocurrent and facies distribution data, which delineate two sources of sediment to the Dunkard basin: a dominant southeastern source and a subordinate northern source. Northern sandstones are rich in nonundulatory, monocrystalline quartz, polycrystalline quartz, and sedimentary lithic fragments. These sandstones largely reflect a stable cratonic source wherein rocks were formed from multicycle sediments, some of which were ultimately derived from a low-lying craton of granite and/or high-rank metamorphic Precambrian rocks. Southern sandstones are enriched in mud rock and foliated, quartz-mica rock fragments, unstable polycrystalline quartz, and polycyclic, monocrystalline quartz reflecting a southeastern source terrane of mixed low-rank metamorphic and sedimentary lithology. Comparison of the Dunkard detrital mode with modern sands from tectonic settings capable of producing uplifted terranes of supracrustal source rocks reveals that the Dunkard basin was a foreland basin that received sediment from both the adjacent foreland fold-thrust upland and from positive areas on the craton. Low to medium-grade metamorphic and recycled sedimentary detritus, shed from an orogenic highland, diluted contributions of supermature sedimentary and magmatic detritus from the continental lowlands. End_of_Article - Last_Page 1446------------

Provenance of the Middle Ordovician Blount clastic wedge, Georgia and Tennessee

Convergent tectonism along the Appalachian continental margin in Middle Ordovician time resulted in cratonward-prograding clastic wedges. Detrital modes of 52 sandstones of the Blount clastic wedge in Georgia and Tennessee are dominated by monocrystalline quartz (68%), feldspar (10%, plagioclase >K-feldspar), and pelitic rock fragments (10%), with lesser amounts of polycrystalline quartz (6%), chert (2%), low-grade metamorphic rock fragments (2%), and quartzofeldspathic rock fragments (0.3%). The primary source rocks were sedimentary; subordinate contributions were from low-grade metamorphic and plagioclase-rich plutonic and/or gneissic rocks. The composition of sandstones from the Martinsburg clastic wedge, based on point-counting 18 samples from the collection of McBride, and from the Taconic clastic wedge, based on data of Hiscott, is similar to the Blount clastic wedge except that Martinsburg and Taconic sandstones have additional evidence of derivation from intermediate or mafic volcanic and deep-water sedimentary source rocks, which were lacking in the Blount source terrane. The differences in provenance among the clastic wedges may indicate along-strike variations in tectonic style, or variations in the distribution of rock types, or differences in the level of erosion within orogenic terranes of similar origin.

The Effect of Grain Size on Detrital Modes: A Test of the Gazzi-Dickinson Point-Counting Method

ABSTRACT Differing methods of determining detrital modes of sand/sandstone have been developed by different schools due to different goals and different geologic settings. The Gazzi-Dickinson method of point counting was developed to maximize source-rock data, while minimizing the time, effort, and expense of gathering such data. Use of the method minimizes variation of composition with grain size, thus eliminating the need for sieving and multiple counts of different size fractions. Unsorted samples of any sand size may be used, thus allowing direct comparison between modern sands and poorly sorted ancient sandstones. The application of actualistic petrologic models relating composition to tectonic setting thus is facilitated. The unique aspect of the Gazzi-Dickinson method of point counting is the assignment of sand-sized crystals and grains within larger fragments to the category of the crystal or grain, rather than to the category of the larger fragment. In addition, every attempt is made to reconstruct original detrital compositions in spite of subsequent alterations. Six unconsolidated Holocene sand samples derived from a variety of source rocks in north-central New Mexico were collected, sieved, impregnated, sectioned, stained and point-counted, using both traditional and Gazzi-Dickinson methods. Results of these counts provide a comparative test of traditional and Gazzi-Dickinson methods. There are two reasons for variation of modal composition with grain size: 1) the breakage of fragments into constituent grains, and 2) actual mineralogic variation with grain size. The Gazzi-Dickinson method successfully eliminates the first source of compositional grain-size dependency. No point-counting method eliminates the second source. Use of the Gazzi-Dickinson method on unsorted samples produces results that are consistent with those from different size fractions of the same samples. Lithic-fragment compositions (for example, LmLvLs, QpLvmLsm) are especially consistent and provide the most useful parameters for relating composition to source rock and, ultimately, to tectonic setting.

Exceptions to the Relationship Between Plate Tectonics and Sandstone Composition

ABSTRACT Routine use of QFL, QpLsmLvm, and LmLsLv plate-tectonic provenance diagrams has led to the recognition of error populations. For these sandstones, the interpretation of tectonic setting based on data exclusive of petrography does not coincide with the interpretation of tectonic setting based on their location on compositional diagrams. The anomalous sandstones can be separated into at least four categories: i) sandstones deposited during the transition between tectonic regimes may be derived, in part, from relict source rocks; ii) the relative enrichment of sandstones in detrital quartz by weathering and/or depositional reworking may lead to an inaccurate interpretation of tectonic setting from compositional data; iii) sandstones deposited in tectonic settings as yet unrepresented on he provenance diagrams may plot between the provenance fields or overlap existing fields; iv) the role of detrital carbonate rock fragments directly affects the location of data points on provenance diagrams, and is an unresolved problem. The first two categories represent permanent problems in the use of sandstone composition as an indicator of plate-tectonic setting, whereas the last two may only be temporary problems that can be solved by more data and a consensus on determining detrital modes. Recognition of anomalous sandstones in the rock record will reduce the probability of erroneous interpretations of tectonic setting and enhance the use of sandstone petrology as an indicator of plate-tectonic setting.

Detrital modes of upper Paleozoic sandstones derived from Antler Orogen in Nevada; implications for nature of Antler Orogeny

Detrital modes of upper Paleozoic sandstones derived from Antler Orogen in Nevada; implications for nature of Antler Orogeny

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.

Deep-sea fan sediments in the Torlesse zone, Lake Ohau, South Canterbury, New Zealand

Abstract Late Triassic Torlesse metasediments outcropping in and around the Lake Ohau Ski-basin comprise interbedded granule-conglomerate, sandstone, siltstone and mudstone. The strata dip steeply and are broken by numerous faults. The sediments have undergone low grade regional metamorphism of the prehnite-pumpellyite facies. Seven facies are recognised within the sedimentary pile. Facies A-F are thought to have been deposited dominantly by one of the processes of gravity-induced subaqueous mass transport, inchiding synsedimentary slumping and debris, inertia, or turbidity flows. Facies G includes finer sediments attributable to hemipelagic rain on an ocean floor. Associations of these facies suggest that sedimentation occurred on and around the midfan region of a submarine fan system. Suites of facies associations reflect the meandering and progradation of the main axes of sediment dispersal over the fan complex. Paleocurrent and paleoslope indicators support a fan model, and possibly suggest turbidity current transport down a northwest-facing paleoslope. Chemical analyses and detrital modes of the sand lithologies require the original source area to be typically acid-plutonic.

Detrital modes of recent deep‐sea sands and their relation to tectonic setting: a first approximation

ABSTRACTComplete petrographic analyses of modern deep‐sea sands are surprisingly rare, but are needed if problems such as the influence of tectonics on petrology, or changes in petrology due to diagenesis, are to be understood in ancient sandstones. This paper is a first attempt, using data from our own study of 80 sands plus data from the literature, to establish a baseline for modern deep‐sea sands with which ancient sandstones can be compared. We assigned sands to five plate‐tectonic categories, each of which has a distinct average petrologic composition: (1) basins associated with trailing‐edge continental margins, Q62F26L12; (2) with leading‐edge continental margins, either subduction, Q16F53L31, or (3) strike‐slip, Q34F39L27; (4) back‐arc basins, Q20F29L51; (5) fore‐arc basins, Q8F17L75. The amount and type of lithic fragments and the type of feldspar are particularly important discriminators, the amount of quartz less so. Trailing‐edge and fore‐arc basins have quite distinctive compositions, but back‐arc basins and basins near the two types of leading‐edge margins show some overlap, and thus may be hard to differentiate in ancient sandstones.

Detrital Mode Evolution of the Rifted Continental-Margin Longobucco Sequence (Jurassic), Calabrian Arc, Italy

ABSTRACT The Longobucco Sequence is a 1200-m-thick dominantly carbonate miogeoclinal wedge which accumulated in the Mediterranean area during Jurassic time. Sedimentary facies pass upward from continental (3% of the stratigraphic column) at the base, through shelf (6%), slope (14%), to deep-sea turbidites (77%) at the top. Sand framework grains had five sources, four are noncarbonate extrabasinal and one, the most abundant, is carbonate intrabasinal as follows: reworked arenites (quartzarenites, petrofacies A); low-rank metamorphic rocks (litharenites, petrofacies B); eolian or beach arenites (feldspathic-quartzarenites, petrofacies C); granitic and high-grade metamorphic rocks (arkoses, petrofacies D); intrabasinal shelf sediments (peloids, intraclasts, fossils and oolites). Quartzarenites, which form fluvial and shallow-marine deposits, gradually pass upward through shelf, siliciclast-rich limestone to marly slope deposits with minor interbedded litharenites. Deep-sea fan deposits are chiefly carbonate turbidites, but feldspathic quartzarenites are present in the lower part. Arkoses abruptly replace the feldspathic quartzarenites in the upper part of the section. The litharenitic source is represented throughout the stratigraphic column. Therefore, different terrigenous source areas simultaneously supplied detritus to the sedimentary basin. The evolution of detrital modes and depositional systems suggests two major tectonic events: latest Early Jurassic and probably Late Jurassic, superimposed on a progressive sinking of the basins. Both the change in depositional systems and sand composition can be related to a complex interplay of microcontinents and oceans which determined local compressive or extensional regimes during Alpine rifting in the Tethys region.

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------------