Valley Rock Research Articles

Early diagenetic microporosity in the Cotton Valley Limestone of east Texas

The Upper Jurassic, Cotton Valley Limestone was deposited on a mature ramp where monotonous, regional slopes were punctuated by salt-generated structures and basement topography. The strandline and the crests of paleobathymetric highs were blanketed by oolitic and palletoidal grainstones. The ratio of grainstones to mudstones increases in the upper Cotton Valley, reflecting a regional shallowing phase. Thinner, shoaling-upward sequences are present but they do not correlate easily, especially from basement highs to salt domes, probably because salt movement occurred during deposition of Cotton Valley rocks. The complex diagenetic history of the Cotton Valley, inferred from cross-cutting features observed in thin section and from trace-element and stable-isotope content, includes the origin of “chalky” microporosity, especially in ooids. This intraparticle, intercrystalline porosity occurs in a fabric of equant, subhedral to euhedral, low-Mg calcite micro-rhombs which appear to have developed at the expense of an acicular precursor. Such microporous ooids are present mainly on the crests of paleobathymetric highs; nearby, offstructure ooids contain a mixture of micritic and well-preserved ooids. However, these micritic ooids are different from the micro-rhombic, microporous ones on the highs. The well-preserved low-Mg calcite ooids from offstructure positions exhibit relict acicular microstructures in some of their lamellae. The “chalky” microporosity is crosscut by virtually every other diagenetic feature in the Cotton Valley Limestone. The ϵ 13C values from individual microporous ooids range from +1.65 to +2.76% PDB, which is not in the range of values associated with precipitation in a hydrocarbon-rich environment. The formation of microporosity was followed by the formation of embayed grain contacts, pore-filling cementation, grain compaction, stylolite formation, replacements by quartz and rhombic dolomite, fracturing, fracture-filling cementation, saddle dolomite, late leaching, and formation of authigenic sulfides. The microporosity is interpreted to have been formed in the near surface diagenetic environment, early in the burial history of the Cotton Valley. As there is no evidence of vadose diagenesis, the ooids on the crests of calcarenite shoals must have been placed is disequilibrium with their surroundings by a change in water chemistry, probably as a consequence of regression and an influx of fresh water. The introduction of hydrocarbons appears to be contemporaneous with such late diagenetic features as saddle dolomite and authigenic sulfides; however, the extent to which those fluids affected the micro-rhombic calcite crystals appears to be negligible.

Indus Valley rock carvings under threat from road-building plans

Indus Valley rock carvings under threat from road-building plans

Facies History of the Upper Jurassic-Lower Cretaceous Great Valley Sequence: Response to Structural Development of an Outer-Arc Basin

ABSTRACT The facies history of the Great Valley sequence is closely linked to structural events in an arc-trench system. Upper Jurassic-Lower Cretaceous marine rocks, as much as 8,500 m thick along the western margin of the Sacramento Valley, northern California, were deposited on slopes and submarine fans that extended over an unstable basement floored by ophiolitic rocks. Paleocurrent and paleoslope analyses indicate that eastward subduction, initiated as early as Kimmeridgian, created an outer-arc ridge which bounded the west side of the basin. Debris was shed dominantly from the north and east, but significant contributions were ridge-derived. Sediment fabrics, structures, and bedding styles indicate that earliest deposition on basin plains and on eastward facing slopes was followed, at le st until the Valanginian, by a stable sedimentological framework of prograding and avulsing submarine fans characterized by broad, shallow midfan channels, depositional lobes, and abundant, thick deposits of interchannel mud. The Great Valley rocks are transected by northwest-trending faults that produced strike separation of the east-dipping strata. Smooth facies transitions, but with abrupt thickness variations across faults, suggest that faulting with some normal component of motion was contemporaneous with deposition during latest Jurassic and earliest Cretaceous. Later movement, either strike-slip or reverse, brought coarse, inner-fan facies that were deposited on Klamath basement into fault contact with basin-plain facies that were deposited on ophiolitic basement.

Controls of Deformation Mechanisms and Fracturing on Local and Regional Hydrocarbon Potential in the Central Appalachian Overthrust Belt: ABSTRACT

In highly deformed fold and thrust belts, strain associated with different deformation mechanisms can significantly alter the porosity and permeability of potential reservoir rocks. In general, mechanisms such as pressure solution, intragranular deformation, and cataclasis reduce porosity, whereas extension fracturing increases porosity, and more importantly, permeability. Since these mechanisms are dependent on parameters such as temperature, pressure, deviatoric stress, lithology, and grain size, they can produce significant regional and local variations in reservoir potential. Thus studies of these mechanisms are important for defining regional limits for exploration and, more locally, for evaluating individual prospects. In the central Appalachians, exploration is presently confined to the Plateau and Valley and Ridge provinces. However, the regional limits of hydrocarbon potential need to be defined, particularly in light of the recent hypothesis by Harris et al in 1981 of subthrust potential underlying the Blue Ridge in the southern Appalachians. This can be done by combining data on strain and deformation mechanisms with conodont CAI data and estimates of displacements on major faults. Penetrative strain axial ratios (R) range between 1.1 and 8.0 in Great Valley carbonates and between 1.6 and 6.9 in Blue Ridge clastics. In the Valley and Ridge, R is usually less than 1.2. For most reservoir rocks, porosity is completely eliminated for R = 1.5, so that little porosity is expected east of the Valley nd Ridge. Conodont CAI data of Epstein et al in 1977 show values approaching 5.0 for the Cambro-Ordovician carbonates and 4.5 for the Silurian-Devonian carbonates along the eastern edge of the Valley and Ridge. These values are at the upper limit for commercial gas production. The North Mountain and Blue Ridge faults, the two major thrusts east of the Valley and Ridge, both have maximum estimated displacements less than 20 km (12 mi), so that subthrust Valley and Ridge-type Silurian-Devonian rocks underlying the Great Valley rocks are fairly limited. Subthrust Cambro-Ordovician carbonates may be more widespread under the North Mountain ramp, but their potential is low because of high CAI and R values. The eastern limit of potential is therefore marked by the North Mountain fault, except or limited areas of subthrust Silurian-Devonian rocks. The local effects of deformation mechanisms are related to interactions between pressure solution and fracturing, defined by relative timing, lithological characteristics, and structural position. Longitudinal fractures formed early or during the main phase of deformation are often healed or sealed by pressure solution, while transverse fractures, particularly those formed late tend to remain open. Lithological types susceptible to pressure solution, such as impure limestones, are less likely to retain matrix or fracture porosity than pure orthoquartzites. Structural positions with dilational strain, such as fold hinges in competent units, retain more porosity than those undergoing plane strain, such as thinned forelimbs of folds. Brittle deformation zones along splay faults are typic lly sealed through cataclasis. Consideration of these factors can significantly improve the evaluation of prospects in overthrust belts. End_of_Article - Last_Page 516------------

Chertification in the Mississippian Lake Valley Formation, Sacramento Mountains, New Mexico

ABSTRACTChert distribution in the Lake Valley rocks is selective to mud‐supported facies; it is not related to proximity to unconformities. The facies selectivity of the chertification is believed to be a function of the depositional distribution of indigenous silica as sponge spicules, an interpretation that is supported by high positive qualitative correlation of chert with spiculitic rocks. Petrography indicates that the spicules were all originally siliceous, and that they all went through a moldic stage during which many molds were compactively destroyed and distorted. Remaining molds were subsequently cemented by calcite or chalcedony. Chert distribution and spicule petrography argue for an intraformational source for much of the silica.Chert micro‐fabrics are dominated by microquartz, a replacement of grains and lime mud; length‐fast chalcedony, a pore‐filling cement; and megaquartz, a post‐chalcedony pore‐filling cement.Petrography of compaction features within chert masses indicates that chertification occurred after some burial. Based on stratigraphic reconstruction this burial depth was a maximum of about 215 m. and was most likely a few metres to a few tens of metres.Petrography of chert‐calcite cement relationships indicates that chertification occurred before and during first generation, pre‐Pennsylvanian non‐ferroan calcite cementation, and was completed before late‐stage, post‐Mississippian ferroan calcite precipitation. Petrography of chert clasts in basal Rancheria (Meramecian) and basal Pennsylvanian conglomerates proves these clasts derived from the Lake Valley Formation and were chertified before redeposition. Thus, some cherts in the Lake Valley are pre‐Meramecian in age, but all are pre‐Pennsylvanian in age. Furthermore, association of the cherts with the non‐ferroan cements indicates the cherts were probably precipitated in meteoric phreatic lens established beneath the pre‐Meramecian and pre‐Pennsylvanian subaerial unconformities.

Endolithic Blue-Green Algae in the Dry Valleys: Primary Producers in the Antarctic Desert Ecosystem

Endolithic unicellular blue-green algae occur under the surface of orthoquartzite rocks in the dry valleys of southern Victoria Land, Antarctica. This report of primary producers in the Antarctic desert ecosystem suggests that, in future efforts to detect life in extraterrestrial (for example, martian) environments, scientists should consider the possible existence of endolithic life forms.

Anatomy of an Orogen

Mapping a strip of 15′ quadrangles across the northern Coast Ranges of California by student colleagues and myself has resulted in the development of a model for the origin and emplacement of the Franciscan complex and its relation to the coeval Great Valley sequence that differs from earlier models. Rocks of both Franciscan and Great Valley are predominantly graywacke, silty mudstone, and local conglomerate. Most of the Great Valley rocks are graded, and bottom markings are common; these features are present but not characteristic of the Franciscan rocks. Franciscan graywacke and silty mudstone tend to be broken and sheared, or chaotically deformed. The chaotic zones typically contain exotic blocks, mostly of ophiolitic rocks, but also blue-schist, mica schist, metagraywacke, and blocks similar to the enclosing sheared matrix. These chaotic units are best characterized as melanges. Within the Franciscan complex, mappable units have been defined, based on lithology of matrix and content of exotic blocks. The units dip and face easterly and are oldest to the east (Tithonian or older for the South Fork Mountain Schist). Progressively younger rocks occur down section to the west; the youngest are the Coastal Facies largely of Eocene age. The probable origin involves deposition of Franciscan sediments in a trench, followed by thrusting of the sediments against and under the landward side of the trench, while the axis of the trench migrated progressively westward. The process began in Tithonian time, slowed during Late Cretaceous, and ceased in the Eocene. Models are suggested for the formation of melanges during sedimentation and underthrusting. Each melange unit contains a unique assemblage of exotic blocks. It is suggested that these assemblages in part reflect the lithology of the basement rocks on which the sediments were deposited. If true, the composition of the basement varied from dominantly ophiolite-suite rocks to dominantly blueschist. A melange of ophiolite blocks also formed, the basement beneath a part of the Great Valley sediments. Samples of radiolarian chert from the ophiolite suite throughout the Franciscan were determined to be of Tithonian age by Emile Pessagno. If the ophiolite represents ocean floor, formed at a spreading ridge, the ridge must have been located adjacent to the trench in Tithonian time; after the Tithonian, no oceanic rocks were added to the Franciscan pile. A diapiric origin for the Tithonian ophiolite is suggested. Northwest-trending isolated slabs of Great Valley—type rocks and of shallow-water to continental coal-bearing clastic rocks of Cretaceous to Miocene age are scattered within the Franciscan complex. Evidence is presented that these were deposited in separate, probably fault-controlled basins, developed on the surface of the tectonically active Franciscan complex during and following underthrusting. They do not appear to represent a klippen of a Coast Range thrust fault, as has been suggested by others.

Cretaceous Sedimentation and Regional Thrusting, Northeastern Diablo Range, California

Stratigraphic and structural details in the juxtaposed Franciscan and Great Valley Sequences of the northeastern Diablo Range, California, allow restriction of the age of regional thrusting or gravity sliding. The Cretaceous section in the area consists of three sandstone-shale units: the Albian-Turonian Adobe Flat Shale, the Coniacian-Campanian Panoche Formation, and the Campanian-Maestrichtian Moreno Formation. Both the upper and lower parts of this section are absent locally, having been removed by erosion or faulting, or both. The middle unit, the Panoche Formation, varies in thickness. About 18,500 ft of Coniacian-Campanian Panoche section are exposed in Del Puerto Creek on the south, and a similar thickness of equivalent rocks is present in the Byron-Tesla area on the north. Between these areas, near Hospital and Ingram Creeks, the Panoche section, deposited over the same time range (Coniacian to late Campanian), has a maximum thickness of only 9000 to 10,000 ft. Although some of the section has been removed by faulting, much thinning has apparently resulted from differential deposition over a topographic high. This topographic high corresponds geographically with the location of the present Mount Oso Anticlinorium located in Franciscan rocks to the southwest. These relations imply that the Mount Oso Anticlinorium occupied the same relative geographic position during deposition of the Panoche beds. If so, no large-scale tectonic transport between Great Valley and Franciscan rocks, by regional overthrusting, underthrusting, or gravity sliding, has occurred since Coniacian time in the northeastern Diablo Range.

Time of Completion of Underthrusting of Franciscan beneath Great Valley Rocks West of Salinian Block, California

Research Article| September 01, 1970 Time of Completion of Underthrusting of Franciscan beneath Great Valley Rocks West of Salinian Block, California BENJAMIN M PAGE BENJAMIN M PAGE Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar Author and Article Information BENJAMIN M PAGE Department of Geology, Stanford University, Stanford, California 94305 Publisher: Geological Society of America Received: 30 Mar 1970 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1970, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1970) 81 (9): 2825–2834. https://doi.org/10.1130/0016-7606(1970)81[2825:TOCOUO]2.0.CO;2 Article history Received: 30 Mar 1970 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation BENJAMIN M PAGE; Time of Completion of Underthrusting of Franciscan beneath Great Valley Rocks West of Salinian Block, California. GSA Bulletin 1970;; 81 (9): 2825–2834. doi: https://doi.org/10.1130/0016-7606(1970)81[2825:TOCOUO]2.0.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 Underthrusting of the Franciscan assemblage beneath Great Valley rocks seems to have been completed by the end of the Oligocene in the region west of the Salinian block. Both the Franciscan and Great Valley assemblages are latest Jurassic-Cretaceous in age. Clasts derived from both first appear together in the Vaqueros Formation (Upper Oligocene?-Lower Miocene). The Vaqueros is markedly transgressive and is generally the oldest formation in depositional contact with the Franciscan in the region under discussion. The Vaqueros locally rests on both Franciscan and Great Valley rocks, and extends across a fault separating the two.If Underthrusting ceased before the end of the Oligocene, the timing is compatible with the termination of northeast-southwest relative motion at the continental margin, and the beginning of northwest-southeast relative motion when North America encountered the Main Pacific plate. Much or all activity on the San Andreas fault followed this event. The fault offset the zone of Underthrusting. 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.

Developments in Arkansas, North Louisiana, and East Texas in 1966

The Arkansas, North Louisiana, and East Texas district had a 7% increase in new-field discoveries during 1966, although exploratory drilling decreased 15%. Development drilling declined 36%, and seismic exploration declined 37%. Exploratory activity was highest in Arkansas, which had the largest drilling program since 1961, a 41.5% increase in wells drilled compared to 1965. An intense search for Smackover production was the primary cause of this increase. This activity is expected to continue in 1967, and, combined with the planned drilling of a 15,000-ft. Paleozoic test in Ashley County by Humble, should give Arkansas another good exploration year. Independent operators conducted most of the exploratory activity in North Louisiana. The Wilcox Group and the Sligo Formation were the chief targets, and the search for oil in these beds is expected to continue in 1967. Exploratory activity in East Texas was essentially in the Smackover trend, but during the year more emphasis was placed on drilling to the Cotton Valley and Paleozoic rocks. It is anticipated that these formations will be explored further during 1967.

Occurrence of ignimbrite in the Shag Valley, North-east Otago

Summary Petrography of the acid igneous rock of the Shag Valley in North-east Otago, previously classified as a quartz porphyry, indicates that this rock is an ignimbrite. Formerly described as a dyke, the Shag Valley rock is now considered to be a surface rock interbedded with coal measures of Upper Senonian age, and is the first occurrence of ignimbrite described from the South Tsland of New Zealand.

K-Feldspar Content of Jurassic and Cretaceous Graywackes of Northern Coast Ranges and Sacramento Valley, California

Graywackes of Late Jurassic to Late Cretaceous age are the predominant rocks in the northern Coast Ranges and the western Sacramento Valley provinces in California. These graywackes are similar in appearance, but their content of K-feldspar, which can be readily estimated after selective staining, differs widely. Study of several hundred specimens indicates that K-feldspar content is helpful in correlating or distinguishing these rocks, and sheds light on the geologic history of western California. In the Sacramento Valley a conformable sequence of Knoxville (Upper Jurassic), Shasta (Lower Cretaceous), and Upper Cretaceous rocks progressively increases in average K-feldspar content with decreasing age. Graywackes of the Franciscan formation, lying in the Coast Ranges farther west, generally contain no K-feldspar, although some contain a trace, and a few of more uncertain affiliation contain large amounts. Another belt of graywackes, west of the Franciscan formation and paralleling the Pacific shore, contains nearly as much K-feldspar as the Upper Cretaceous rocks of the Sacramento Valley, and most of these rocks are probably of middle Cretaceous age. The writers attribute the increase of K-feldspar with decreasing age of the rocks of the Sacramento Valley to changes in the source area, which is believed to be the Klamath Mountains and the Sierra Nevada. The changes include an increase in proportion of granitic rocks to others as the areas were stripped, and more abundant K-feldspar in the younger granitic rocks. The high K-feldspar content in the middle Cretaceous rocks of the coastal belt suggests that the rocks were deposited in the same basin as the Sacramento Valley rocks.

The Virginia earthquake of April 9, 1918

Summary of geology and conclusions Since, from the reports received, the shock seems to have been most severe in the northern part of the Valley of Virginia, a very brief summary of the geology of the northern valley region is of some interest in seeking the probable cause of the earthquake. The Valley of Virginia is bounded on the southeast by the Blue Ridge, the central portion of which is composed of pre-Cambrian igneous rocks and on the northwest by the Valley Ridges subprovince of folded sedimentary rocks ranging up to Devonian and Mississippian in age. The valley maintains an approximate width of twenty miles from the state boundary southwestward to nearly the latitude of Greenville, Augusta County (Map, Plate I). From near the latitude of Strasburg and Riverton to that a short distance south of Harrisonburg, the valley is divided lengthwise by Massanutten Mountain, which is synclinal in structure and composed of sedimentary rock ranging up to and including Devonian in age. The mountain extends southwestward for a distance of about forty-five miles, and divides the valley lengthwise into two narrow valleys which average from five to ten miles in width. The Massanutten syncline, however, which involves the Martinsburg shale (Ordovician) at the surface, continues for a considerable distance both to the northeast and to the southwest of the north and south ends of the mountain proper. The valley bottom is developed on folded limestone and shales of Cambro-Ordovician age, underlain by quartzites, sandstones, and shales of Lower Cambrian age which, because of their structure and greater resistance, are exposed along the northwest flank of the Blue Ridge. No igneous rocks are known to occur in the valley proper north of the latitude of northern Rockingham County. The valley rocks are faulted, but in some localities at least the faulting appears to be slight, since the displacement is frequently not great enough to cut one or more formations. Bassler has recognized faulting at Winchester, one of the localities of highest intensity (VI R.-F. scale), during the earthquake of April 9, 1918. He says:5 “Although the full geologic structure in the vicinity of Winchester could not be determined because of lack of continuous exposures, the quarries and other outcrops just west and east of the town indicate that by faulting a band of Lower Ordovician dolomitic limestones has been interpolated between a band of Stones River limestones on the west and argillaceous limestones and shales of Chambersburg and Martinsburg age on the east.” Faulting occurs at the base of Little North Mountain along the northwest side of the valley, and along the northwest front of the Blue Ridge on the southeast side of the valley a great overthrust fault, which apparently follows the Blue Ridge, has a horizontal displacement in places of at least four miles. It seems probable, therefore, that the seismic disturbance of April 9, 1918, had its origin in one or more of the faults which characterize the region.