Composition Of Pebbles Research Articles

Late Middle Proterozoic Hazel Formation near Van Horn, Trans-Pecos Texas: Evidence for transpressive deformation in Grenvillian basement

Research Article| March 01, 1994 Late Middle Proterozoic Hazel Formation near Van Horn, Trans-Pecos Texas: Evidence for transpressive deformation in Grenvillian basement KRISTIAN SOEGAARD; KRISTIAN SOEGAARD 1Programs in Geosciences, University of Texas at Dallas, P. O. Box 830688, Richardson, Texas 75083-0688 Search for other works by this author on: GSW Google Scholar DAVID M. CALLAHAN DAVID M. CALLAHAN 2Golder Associates Inc., 1809 North Mill Street, Suite C, Naperville, Illinois 60563 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1994) 106 (3): 413–423. https://doi.org/10.1130/0016-7606(1994)106<0413:LMPHFN>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation KRISTIAN SOEGAARD, DAVID M. CALLAHAN; Late Middle Proterozoic Hazel Formation near Van Horn, Trans-Pecos Texas: Evidence for transpressive deformation in Grenvillian basement. GSA Bulletin 1994;; 106 (3): 413–423. doi: https://doi.org/10.1130/0016-7606(1994)106<0413:LMPHFN>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The latest Middle Proterozoic Hazel Formation near Van Horn in the Trans-Pecos region of west Texas is an orogenic elastic wedge that provides insight into paleotectonic evolution of the southern margin of the Laurentian craton. The Hazel Formation is >2.5 km thick adjacent to the contemporaneous Streeruwitz thrust fault and consists exclusively of terrestrial sedimentary rocks. Boulder-sized clasts in conglomerate were deposited within alluvial fans north of the Streeruwitz thrust sheet, 0along the southern margin of a basin of unknown geometry and extent. The alluvial-fan conglomerate interfingers with eolian sedimentary rocks in the northern part of the basin. Alluvial-fan conglomerate megasequences indicate that at least two episodes of faulting accompanied Hazel sedimentation. Alluvial megasequences are overall aggradational within the Hazel succession as revealed by clast size variations within the conglomerate.Systematic variation of pebble composition up through the Hazel conglomerate provides an erosional history within the source terrain. Carbonate and subordinate mafic volcanic rocks, derived from the pre orogenic Allamoore and Tumbledown Formations, were the principal source off the Hazel Formation. Metavolcanic and metasedimentary rocks from the Carrizo Mountain Group in the present tectonic hinterland are notably absent in the Hazel conglomerate. Moreover, a rhyolite terrain, presently not exposed in the Van Horn area, was a significant component of the Hazel source.Juxtaposition between source terrain and sedimentary basin, coupled with aggradation of alluvial-fan megasequences along the southern fault-bounded basin margin, supports a transpressive regime for this orogenic belt rather than a purely convergent margin.Deformation associated with the Hazel orogenic sequence post-dated a 1,126- to 1,070-m.y.-old regional felsic igneous event; Grenville deformation in the Llano uplift of central Texas predated the same igneous event. This implies that the Hazel tectonic episode was a second, later Grenvillian deformation. The terms Llano orogeny and Hazel orogeny are introduced here for the Grenville province in Texas and may be comparable with the Elzevirian and Ottawan orogenies within the Grenville province of Canada. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geochemical and isotope composition of pebbles from the Caban Conglomerate Formation and their bearing on the source of Welsh Palaeozoic sedimentary rocks

AbstractMajor, trace and REE geochemistry, together with Nd isotope determinations, provide new insights into the provenance of seven pebbles of igneous origin from an early Silurian facies of the Caban Conglomerate Formation from central Wales. The geochemical composition of the pebbles supports formation in a subduction related environment on an active continental margin. Biotite whole-rock Rb-Sr systematics constrain the age of a granite pebble to 650±38 Ma. The pebbles provide discrete examples of Avalonian basement which contributed to the isotope composition of sedimentary rocks in the Welsh Basin and one granite pebble provides a rare example from the Welsh Basin of an igneous rock with a Palaeoproterozoic depleted mantle model age.

Nearshore Conglomerates of Vaqueros Formation, Westernmost Santa Ynez Range, California: ABSTRACT

The Vaqueros sequence in this area can be divided into three lithofacies. The basal facies (facies A) consists of inversely graded, matrix-supported, boulder to pebble-conglomerates. These conglomerates are angular, nonfossiliferous, and laterally persistent. The middle facies (facies B) is characterized by nongraded to normally graded, sandy, cobble to pebble-conglomerate. Pectens, oysters, gastropods, and clams are abundant in this facies and individual beds have sharp bases and burrowed tops. Small-scale lenticular bedding and reworked lenses of fossil material are common throughout. Facies C (the upper facies) is dominated by extensively bioturbated, locally cross-bedded, pebbly coarse sandstone. Pebble imbrication throughout the sequence, channel orientations in facies B, and cross-bedding in Facies C record paleoflows to the east and the southeast. Pebble lithologies and sandstone composition indicate a dominant Franciscan-assemblage provenance for the entire sequence. These facies represent deposition in proximal (facies A) and distal (facies B and C) fan-delta and braid-delta environments. Facies A conglomerates record braided fluvial deposition, facies B conglomerates record channel-mouth deposition, and the sandstones of facies C record deposition in shoreface to inner shelf environments. The sequence of strata documents local sedimentary response to complexly intertwined tectonic and eustatic events. The deepening-upward sequence described here was deposited onmore » the southern flank of the then-emergent Santa Maria basin (the Paleotrench-slope break.) during a late Oligocene-early Miocene eustatic sea level rise and suggests an eastward dipping paleoslope for the western margin of the late Oligocene-early Miocene Western Transverse Range province.« less

Late Proterozoic glaciation in central-eastern Brazil: Synthesis and model

In the surroundings of the Middle Proterozoic São Francisco Craton of central-eastern Brazil, numerous traces of an ancient glaciation can be found. Almost all criterua necessary to prove an ancient glaciation occur, including scratched and facetted pebbles, a striated surface, quarrying, outwash plains, eskers, varvites, dropstones, “décollement” structures etc. Poorly sorted tillites with varying pebble composition are widely distributed. Continental tillites and glaciomarine sequences interfinger in the east and southeast of the craton. Traces of ice movement suggest flow directions of the ice towards the east and the southeast of the craton. At the coast, the wet-base glaciers passed over to floating shelf ice. In the northeast, glaciers also came from the Serrinha Craton in northeastern Bahia. The glaciation covered an area of at least 1300 × 700 km. The sequences containing traces of glaciation discordantly overlie Archean to Middle Proterozoic rocks. They in turn are overlain by Late Proterozoic calcareous and psammopelitic sediments of the Bambuí Group. Present data indicate that glaciation took place around 1000 Ma ago.

Application of till pebble lithology to the differentiation of glacial lobes in southern Alberta

Variations in the pebble composition of surface till in southern Alberta indicate the presence of three contemporaneous lobes in the last continental glacier. The ice lobe domains have similar pebble associations and differ only in the frequency of occurrence of rock constituents. A large part of the percentage variability within each domain is caused by irregular entrainment of locally derived stones, which dilutes pebble contents and results in a lithology substantially different from the provenance composition. The study has shown that the regional trends are preserved and can be recognized in the locally modified lithologies, but the latter have to be isolated and evaluated differently from the regular provenance lithologies. The common diluting agent in the area is fluvial gravel that is widespread on erosional bedrock surfaces and in preglacial valleys. Because the gravel is generally coarse, pebble fractions of larger size are more affected by local dilution than are successively finer fractions. Quantitative comparison of rock contents in two size ranges helps to identify the regional and local types of pebble composition and to segregate petrological data into two corresponding sets. Source indices are calculated then as a function of three lithologic variables, for each of the sets separately. The indices have similar values in nondiluted and diluted samples of a given area and distinguish well between tills deposited by glaciers with different directions of regional flow.

Mid-Tertiary Conglomerate Deposition and Structural Implications, Southwestern Wind River Range, Wyoming: ABSTRACT

Study of a granule-to-boulder conglomerate of probable middle to late Eocene age provides evidence that the last pulse of uplift of the Wind River Range occurred along a fault or fault zone within the crystalline core of the range. The conglomerate is preserved on the northeast side of the Continental fault between Tabernacle Butte on the west and Horsetrack anticline on the east. It overlies rocks of the Bridger Formation (Eocene), locally overlies Precambrian crystalline rocks, and appears to be overlain by lower Miocene strata. The conglomerate consists of numerous coarse pebbles, cobbles, and boulders floating in a matrix ranging from coarse sand to granules. Many of the nonequidimensional pebbles and cobbles are imbricated and isolated, suggesting current scour on the upstream side of the pebbles, and are oriented with their long axes between transverse and longitudinal positions, indicating later reworking of orientations developed during initial high velocity flows. Thus, fabric indicates deposition in a fluvial system having highly variable current velocities. Distributary paleocurrent patterns, proximity to a major uplift, and the presence of large boulders suggest deposition on alluvial fans. These alluvial fans were characterized by deposition of moderately to poorly sorted coarse sand to boulder-size material in a braided stream environment. Although the conglomerate is matrix supported, deposition by fluvial processes is indicated by well-defined stratification, lenticularity of bedding, common scour surfaces, local cross-stratification, channel fills ranging up to 31 m (102 ft) wide, and fabric as noted above. Evidence of mass flows is very rare in this unit. Composition of pebbles and cobbles in the conglomerate is quite variable, indicating both the complex Precambrian geology and the erosion of Precambrian units within the core of the range as well as those exposed locally. The large amount of finer sediments (sand and granules) in the conglomerate at exposures near the Precambrian paleohighs suggests, at least in part, a distal source. Mapping of other Eocene sedimentary deposits in the Big Sandy Opening area suggests the presence of fluvial tributaries well within the crystalline core of the range. These characteristics suggest a middle or late Eocene pulse of uplift within the core of the range resulting in incision of tributary systems into the Precambrian rocks of the southwestern margin of the Wind River Range. A decrease in strea gradient at the southwestern limit of the uplifted crystalline rocks resulted in deposition of much of this core-derived sediment on basin-margin alluvial fans. This system was probably morphologically similar to but smaller in scale than, the modern Kosi fan system of northeastern India. End_of_Article - Last_Page 1331------------

Investigations Bearing on the Age and Development of Chesil Beach, Dorset, and the Associated Area

Recent site investigations and research on Chesil Beach, Dorset, have provided new data which help to shed light on the origin of both the Beach and the lagoonal Fleet which is situated to landward. A channel of the order of 26 m is indicated between the Isle of Portland and the mainland. Under much of the Fleet there is a marked break of slope at about I5 m O.D. This is associated with pebbles, cobbles and boulders and takes the form of a storm beach with planation surface to seaward. Overlying this surface are various deposits dating from Pollen Zone VI onward. In the light of these data the surface must, as in the case of those at Orford, Suffolk, be entirely preFlandrian. A tentative chronology is given. THE purpose of this paper is to discuss aspects of the evolution of Chesil Beach in relation to the Fleet and the Portland Raised Beach. The paper describes certain facets of recent work carried out in the area by the former Physiographical Section of the Nature Conservancy and incorporates new data from elsewhere. Chesil Beach is one of the three major shingle structures on the coast of Great Britain and the only one without a cuspate form of development. The beach extends at least 8 km from Chesilton in the east, where it terminates against the cliffs of the so-called Isle of Portland, to an essentially arbitrary limit in the west. (For location of most places referred to in the text, see Figure ia.) At both ends the shingle structure is joined to the mainland but over the remaining 13 km it is backed by the shallow, tidal, Fleet. The latter varies in width from under ioo m to some 900 m, and in depth from less than -0 3 m to nearly -3-0 m O.D. (Ordnance Datum, Newlyn, is approximately mean sea level at this site.) Opposite the Fleet, Chesil Beach is between I50 and 200 m wide but it is narrower both adjacent to the cliffs in the west and at the extreme eastern end. The crest is broken intermittently at the western end but extends continuously from just east of Section 12 in Figure ia to Chesilton. While minor irregularities of the order of 0-5 m, and occasionally more, occur throughout this distance, the broad picture is of a progressively increasing ridge height from west to east, the maximum of I4'7 m O.D. being found near Chesilton. Pebble size above low water mark also increases in the same direction. Pebble composition is overwhelmingly of flint and chert (A. P. Carr and M. W. Blackley, I969). Offshore, the beach drops at a gradient broadly similar to that of the seaward face above low-water

Petrochemistry of early Precambrian metasediments from the central part of the Chitaldrug schist belt, Mysore, India

Petrological and geochemical studies of the metasediments in the central part of the Chitaldrug schist belt, Mysore, India indicate that these metasediments, most probably were supplied from an area of predominantly basic composition. Their partial metasedimentary and plutonic provenance is also suggested by the pebble composition of the conglomerates and the modal and chemical compositions of their matrix and associated graywackes. The existence of metasedimentary rocks in the source area of Dharwar sediments suggests at least one more cycle of sedimentation, orogeny, metamorphism and granitization prior to the main Dharwar cycle (2,600 m.y.). The Dharwars are therefore, probably not the oldest sediments of the Indian Shield as was previously thought. Recent radiometric age data support this conclusion. The amphibolitic xenoliths, so frequently found in the peninsular gneisses, may be the relics of the basic component of the Dharwar basement.

Paleocurrents and Origin of Huronian Lorrain Formation, Ontario and Quebec: ABSTRACT

The Lorrain Formation crops out in 3 areas between Sault Ste. Marie, Ontario, and Ville-Marie, Quebec--Bruce Mines, Whitefish Falls, and Cobalt. It conformably overlies the Gowganda Formation in each area; in the Cobalt area it also locally overlies Archean granite. The Lorrain is overlain by the Gordon Lake Formation. In ascending order, the Lorrain Formation consists of arkose, subarkose, jasper-bearing orthoconglomerate, and orthoquartzite. In the Bruce Mines area the Lorrain is 8,300 ft thick and is divided into 5 members (A-E); in the eastern areas the Lorrain is only 5,000 ft thick and is divided into 3 members (lower, middle, and upper). In each area, mineralogic and textural maturity increases upward; grains in immature arkose at the base are subrounded and 44% feldspar, and grains in supermature orthoquartzite at the top are well rounded and 95-100% quartz. Chronologically, the depositional environments suggested are shallow-water marine, lacustrine, delta fringe, overshelved beach, and high-energy beach. Currents flowing south and southeast deposited most of the sediments; however, in jasper-bearing conglomerate of the Bruce Mines area, cross-bedding and pebble composition indicate a 90° variance in transport direction, and an additional source on the east. Abundant potash feldspar and lack of metamorphic and sedimentary rock fragments in the lower members suggest a plutonic provenance. Subsequent deposition of jasper, chert, and quartz-rich detritus indicates erosion and reworking of Precambrian jaspilites and igneous source rocks similar to those now exposed in the Canadian shield. End_of_Article - Last_Page 850------------

A new method for making pebble counts

Pebble composition, classification, and counting studies are simplified if the pebbles are cemented together in a block and the block is then cut into slices with a rock saw.

Variation in Pebble Composition of Wisconsin Outwash Sediments in the Wabash Valley, Indiana

ABSTRACT Variation in the pebble composition of Wisconsin glacial outwash sediments in the Wabash Valley has been significantly controlled by the influx of rock fragments supplied by the local Paleozoic bedrock. Dolostone, the dominant rock type, and limestone were derived from the Silurian and Devonian formations that crop out in northern Indiana. Limestone was also furnished by the Mississippian formations that are exposed in central and south-central Indiana. Clastics were supplied by the Pennsylvanian formations that crop out in the lower Wabash Valley. Chert was contributed by one or more formations in each of the Paleozoic systems exposed in the Wabash Valley. The igneous and metamorphic rocks, whose ultimate source was the Canadian Shield, were supplied by the gravel fraction of Wiscons n till deposits. This study provides an illustrative example of the concept of progressive dilution.

The Effect of Leaching on Drift Pebbles

How different rocks endure the superficial agencies of leaching and solution may be seen from the composition of pebbles in bowlder clay where this has been subjected to the solvent action of meteoric waters. Many years ago the writer took occasion to collect fifty-eight lots of 1oo pebbles each, from bowlder clay in Louisa County, in Iowa. By collecting pebbles of one size, about one-half inch, and by taking all of this size to be found on a limited space of the clay surface, unconscious selection of different kinds of rock was avoided. Thirty-seven of these samples were taken from places where the drift was in its original condition, unchanged by subsequent weathering or decay. Twenty-one samples were taken from the upper part of the bowlder clay, where it had been more or less leached and weathered. In this region the bowlder clay is overlain by loess. The leaching of the clay sometimes extends five feet under the loess, but more often less than this. From this upper part of the till, calcareous material may be partly or entirely absent, so that there is no reaction for carbonates when acid is applied to the clay. Pebbles of limestone, which were no doubt originally present, have wholly or partly disappeared, and only less readily soluble materials remain. The leaching is most complete in the uppermost part of the till, and from here to the unaltered material below, is a zone in which the leaching is incomplete in varying degree. Limestone pebbles in this zone are either etched on the surface, or else they have suffered partial internal solution and are porous and even crumbling and are often yellow or brown from residual or infiltrated ferruginous material. Below is a table which gives the percentages of the most important rocks noted in the study of the pebble samples collected. In