Detrital Modes Research Articles

Petrography of the Caples terrane of the Thomson Mountains, northern Southland, New Zealand

Abstract Sediments of the Caples terrane in the Thomson Mountains have been mapped in five units, namely Bold Peak, Kays Creek, Upper Peak, Momus Sandstone, and Mt Campbell Formations. Each unit has a distinctive clastic petrography and a diagnostic heavy mineral assemblage, as well as being sedimentologically distinct. The Caples terrane as a whole is quartz poor, and clastic assemblages are dominated by volcanic (andesitic) and plutonic (dioritic) detritus. Bold Peak Formation contains microlitic volcanic and plutonic detritus; Atomodesma limestone and extrabasinal calcareous sediment clasts are minor, but significant, components. Lithologies and proportions of conglomerate clasts differ from those of clasts in the enclosing sandstones. Bold Peak Formation sandstones have an average detrital mode of Q10F15L75. The heavy mineral suite is dominated by hornblende, clinopyroxene, and epidote. Momus Sandstone and Mt Campbell Formations are petrographically indistinguishable. They are dominated by dioritic detritus with subordinate felsitic volcanic clasts and are more quartzose than other Caples terrane units. Detrital modes in both formations average Q35,F15L50. The heavy mineral assemblages are wide ranging and include hornblende, clinopyroxene, zircon, micas, and sphene. Upper Peak Formation detrital assemblages are a mixture of Bold Peak and Momus components. Upper Peak sandstones have an average modal cornposition of Q30F20L50. Heavy minerals are fine grained and the suite is dominated by epidotes, opaques, and micas. Kays Creek Formation is almost entirely andesitic; quartz is rare, and sandstone detrital modes average Q5F?0L,5. Heavy minerals are predominantly epidote and opaques. The principal source of the Caples sediments was probably the Brook Street terrane to the west. Minor contributions may have come from Maitai terrane and possibly Fiordland Complex sources. The source of a minor, but persistent, dacitic contribution is not yet identified. Comparisons between detrital modes of major rock units of the Rangitata Sequence show that a simple twofold subdivision of the sequence is no longer tenable; the eastern quartzofeldspathic Alpine Assemblage is petrographically distinct from the western part of the sequence, which can itself be divided into two units.

Petrology of Holocene Fluvial Sand Derived from Plutonic Source Rocks: Implications to Paleoclimatic Interpretation

ABSTRACT Despite the long and continued interest of sedimentary petrologists in the provenance of ancient sediments, little is known about the detrital modes of Holocene sands of known parentage. As a result it has been impossible to use the present as a key to the past for purposes of provenance interpretation. In order to fill this information gap and to aid in further understanding of the provenance problem, first-cycle fluvial sands derived exclusively from granitic plutons in the relatively humid Appalachian Mountains and in the relatively arid Rocky Mountains in the United States have been studied. Characterization of the climatic influence on these sands is based on the results of detailed petrographic modal-analyses of three or five size fractions of 87 sand samples. Distinctive size-dependent compositional trends exist in the modal percents of the four principal constituents of the sands; these are potentially useful in paleoclimatic interpretation. In all size fractions rock fragments are more abundant in the arid west than in the humid east. Also, though rock fragments decrease in abundance with decreasing grain size in both the west and east, the rate of decrease is much higher in the west than in the east. Feldspars increase in abundance with decreasing grain size in both regions, but the quartz/ feldspar ratio is >1 in the east and <1 in the west. This relation holds for all size classes. Accessory, minerals are much less abundant than the feldspars, but show similar trends. Quartz generally decreases in abundance with decreasing grain size in the east. In the west quartz is more abundant in medium sand than in other size fractions. A schematic model is proposed to depict evolution of sand in arid and humid climates from polymineralic coarse-grained source rocks. The model predicts the relative abundances of the four principal detrital ingredients of first-cycle sands in different size fractions as a function of their relative resistance to weathering. This study also establishes the original mineralogic maturity of first-cycle sands. Tie-lines are drawn between coexisting sand compositions with similar maturity using a modification of the traditional QFR triangular diagram. In addition, trends toward mineralogic maturity within the framework of such a QFR diagram are defined.

Textural and compositional transitional stages between various lithic grain types (with a comment on 'Interpreting detrital modes of graywacke and arkose')

Textural and compositional transitional stages between various lithic grain types (with a comment on 'Interpreting detrital modes of graywacke and arkose')

Detrital modes of new zealand graywackes

Graywacke, used here for any dark indurated sandstone, and argillite are the dominant pre-Tertiary rocks underlying much of New Zealand. The strata, mainly Permian to Jurassic, can be divided into two facies: (1) the western marginal facies of orderly, fossiliferous strata exposed in the New Zealand marginal syncline; and (2) the eastern axial facies of strongly folded and faulted, and partly schistose, rocks that occupy the keel of the New Zealand geosyncline. The two facies are in fault contact along the length of New Zealand. Each facies is 15–20 km thick, and records deposition, probably on oceanic crust, at a rate ≈125 ± 25 m/million years (m.y.) for ≈150 m.y. The western marginal facies apparently was deposited on a rapidly subsiding shelf that was positioned as a major topographic bench between an oceanic trench, site of sedimentation of the turbidites of the eastern axial facies, and a volcano-plutonic arc, probable provenance of the detritus in the two facies. The two facies were metamorphosed at low grade under relatively high pressures and low temperatures, probably in the sub-trench belt of paired metamorphic belts, with the provenance region being the sub-arc belt of relatively low pressure and high temperature. The graywackes of the western marginal facies apparently record a progressive erosional modification of the volcano-plutonic provenance, from a volcanic terrane that shed detritus into lower horizons of the sequence to a plutonic terrane that shed detritus into higher horizons of the sequence as dissection bit into the igneous source rocks of the arc. The most common graywackes of the eastern axial facies are more quartzose than those of the western marginal facies, and may have received plutonic detritus that bypassed the shelf between the arc and the trench. Feldspatholithic “volcanic” graywackes are poor in quartz, rich in pyriboles, bear mainly plagioclase feldspar, and contain mostly volcanic rock fragments. Lithofeldspathic “plutonic” graywackes are relatively high in quartz and mica, bear K-feldspar as well as plagioclase, and contain abundant metamorphic as well as volcanic rock fragments. Analogous paired suites of graywackes may be common in the circum-Pacific region.

Textural and Compositional Transitional Stages Between Various Lithic Grain Types (with a Comment on Interpreting Detrital Modes of Graywacke and Arkose"): ERRATUM"

Textural and Compositional Transitional Stages Between Various Lithic Grain Types (with a Comment on Interpreting Detrital Modes of Graywacke and Arkose"): ERRATUM"

Interpreting Detrital Modes of Graywacke and Arkose

ABSTRACT The determination of detrital modes in graywackes and arkoses, here grouped as subquartzose sandstones, requires special attention to the identification of detrital grain types, as defined operationally, and to the recognition of detrital textures, as opposed to textural elements of diagenetic origin. Accurate detrital modes can yield specific information on provenance that can be gained in no other way. Reserving the terms graywacke and arkose for imprecise field descriptions and abandoning the proportion of matrix as a prime means of classification, subquartzose sandstones can be described adequately using six numerical grain parameters. Three primary parameters summing to 100 permit quantitative serial designation of rock types in the form QxFyLz, w ere Q is total quartzose grains, F is total feldspar grains, and L is total unstable lithic fragments in the framework. Three secondary parameters in the form of ratios permit desirable refinement within each primary parameter: C/Q where C is total polycrystalline quartzose grains, P/F where P is total plagioclase grains, and V/L where V is total volcanic lithic grains. Stable grains, whose sum is the parameter Q, are essentially pure silica in mineralogy, and include both monacrystalline quartz and polycrystalline lithic fragments. The latter are gradational to less quartzose types and difficult to distinguish from felsite grains, which display relict textures of volcanic origin and internal relief owing to their polyminerallic nature. Lithic fragments are subdivided at two levels into our main categories within which are various subcategories: (a) volcanic fragments include felsitic, microlitic, lathwork, and vitric types; (b) clastic fragments include silty-sandy and argillaceous types; (c) tectonite fragments include metasedimentary quartzose types and metavolcanic feldspathic or ferromagnesian types; and (d) microgranular fragments include hypabyssal, hornfelsic, and indurated sedimentary types. Interstitial materials include (a) exotic cements like calcite or zeolite; (b) homogeneous, monominerallic phyllosilicate cement displaying textures indicative of porefilling; (c) clayey detrital lutum called protomatrix; (d) recrystallized lutum or protomatrix called orthomatrix with relict detrital texture| (e) murky, polyminerallic, diagenetic pore-filling called epimatr x, whose growth is accompanied by alteration of framework grains; and (f) deformed and recrystallized lithic fragments called pseudomatrix. Extensive albitization and other alterations complicate the interpretation of detrital modes in many rocks, but potential errors commonly can be avoided by close attention to mineralogy and relict texture except where tectonite fabrics disrupt or transpose the original detrital framework. Most subquartzose sandstones were derived from one. or a mixture, of three salient provenance types: (a) volcanic terranes, yielding feldspatholithic rocks nearly free of quartz, (b) plutonic terranes, yielding feldspathic rocks with few lithic fragments, and (c) uplifted sedimentary and metasedimentary tectonic terranes, yielding chert-grain lithic rocks with f w quartz and feldspar grains except where recycled volcanic or plutonic detritus is abundant. Many voluminous accumulations of subquartzose sandstones near continental margins had as their provenance complex volcano-plutonic orogens representing ancient magmatic arcs analogous to modern arcs associated with trenches.

Stratigraphic Variations in Sandstone Petrology, Great Valley Sequence, Central California Coast

West of the Sur-Nacimiento fault zone in the Southern Coast Ranges of California, strata of the Great Valley sequence are exposed in thrust plates resting tectonically on the underlying Franciscan assemblage as is the case east of the San Andreas fault. Detrital modes of sandstones from different horizons in the local sequence show the same systematic variations with Stratigraphic position as do correlative late Mesozoic strata in the main outcrop belt along the west side of the Great Valley. The similar tectonic setting and sandstone petrology suggest the rocks in the two areas are parts of a single extensive terrane separated by fault offset in the Cenozoic.

Proposed Classification and Terminology of Rocks in the Series Bauxite-Clay-Iron Oxide Ore

ABSTRACT In only a small number of rock types is chemical composition used, up to this time, as the main criterion in their petrographic classification and terminology. A new classification and terminology of rocks in the series bauxite-clay-iron oxide ore based on their quantitative mineral composition is proposed herein. The terms laterite and bauxite are explained not only from their substantive, but also from their genetic, point of view. Laterite is a relatively untransported weathered product, containing free aluminum hydroxide, derived in situ from the parent rock, whereas bauxite is a transported weathered product (detrital mode of occurrence). Both of these rocks, however, have the same mineral composition.