Victor V. Cavaroc

Siliceous Spiculites as Shoreline Indicators in Deltaic Sequences

Lithic and faunal properties and stratigraphic relationships of three Pennsylvanian silica–cemented spiculites in the Appalachian Plateau lead to inferences about the time and specific environmental conditions of spiculite formation. Fauna within spiculite and associated cherty beds indicate a normal Pennsylvanian marine environment, whereas excellent preservation of shells, as well as associated uncompacted silicified coal, indicate early introduction of silica cement. Coal and seat rock closely overlying and underlying the spiculites suggest that the silicified beds represent transgressive elements along a swampy shoreline receiving little detritus. Rapid lateral disappearance of spicule material into thick marine–brackish equivalent detrital strata indicates that sponge colonies were excluded from areas of rapid detrital influx. In the landward direction from paleo–shorelines, spiculites grade into coal and desilicated seat rock which may have provided an immediate nearby source for silica. Absence of spiculite and cherty sediments from Pennsylvanian shorelines where a wide brackish zone separates marine environments from the shoreline suggests that abundant sponge colonies could develop only where marine water closely approached the strand.

Composition of sand released from three source areas under humid, low relief weathering in the North Carolina Piedmont

ABSTRACT The petrographic composition of sand size sediment released by granitic, micaceous metamorphic, and first cycle sedimentary source areas was investigated in a portion of the North Carolina piedmont province. The region is characterized by a moist climate and low relief, resulting in moderate to heavy chemical weathering of the source rocks. Despite significant variation between sample sites within source areas, as well as appreciable in situ degradation of the chemically less stable grain types, mineralogical differences between sands from the three sources can be detected. Strong over-riding composition-sediment size relationships however, tend to obscure the differences. These size correlations, which detectably differ between source areas, appear to reflect a characteristic imprint of the source lithology on the sands released by weathering, and themselves may be useful in differentiating the sands origin.

Late Paleozoic of the Tindouf Basin-North Africa

ABSTRACT The Tindouf Basin was a broad, east-west trending, gently subsiding trough through middle and late Paleozoic time. During the Silurian and Devonian periods, over 4,000 m of marine shales, sandstones, and limestones accumulated. In the early Carboniferous period neritic and barrier facies sandstones were deposited in the northern extremities of the basin. These were succeeded, in mid-Carboniferous time, by a southeasterly prograding fluvio-deltaic complex. Late Paleozoic sedimentation was the result of gentle uplift and concomitant erosion of the Anti-Atlas region, resulting in folding and cannibalism of earlier sediments and low grade metamorphics. The Reguibat and Ougarta structures formed tectonically quiescent subaqueous areas throughout the middle and late Paleozoic. The Tindouf Basin and proximal regions form a substantial part of the Paleozoic of northwest Africa. Therefore, its tectonic and depositional fabric must be rationalized within the framework of any proposed reconstruction of Pangea.

Subsurface Geology of the Pamlico Sound Area, Coastal North Carolina: ABSTRACT

The subsurface of the Pamlico Sound area in North Carolina contains the states thickest drilled succession of Mesozoic-Cenozoic strata. The subsurface geology of approximately 1,800 mi2 of northern Pamlico Sound was investigated using logs and cuttings from seven wells and about 135 mi of multichannel reflection seismic line. Crystalline basement rocks dip about 1.2° east-southeast. The overlying strata, which range in thickness from 5,145 ft (Albemarle Sound area) to 9,860 ft downdip (Cape Hatteras), are not significantly affected by faulting. Jurassic(?) through Eocene sediments all appear to have accumulated within a gently subsiding, marine-shelf environment. Four distinct lithofacies characterize these strata. The basal Unit 1 (La Casitan-Trinitian) reflects a westward transgression of coastal sandstone, outer shelf mud, and carbonate across shallow shelf to lagoonal deposits. Unit 2 (Trinitian-Washitan) is an upward-coarsening detrital sequence which terminates with discontinuous, marine-shelf sandstones that reflect a northerly source. An abrupt transition into marine-shelf mudstones and limest nes marks the base of Unit 3 (Woodbinian-Austinian); these are abruptly overlain by interbedded shelf sandstones that merge to the southwest. Unit 4 (Tayloran-Claibornian), the uppermost interval studied, is characterized by thick, widespread marl, which grades into glauconitic sandstone and finally marine limestone. Continuous seismic reflectors occur at the basement interface and the top of Units 2 and 4. A reflector, possibly a stratigraphic discontinuity, occurs at the top of the Cretaceous in Unit 4. Well-developed prograding seismic sequences occur above Unit 4. End_of_Article - Last_Page 1442------------

Coal Occurrence in Progradational Deltaic Sequence, Raleigh County, West Virginia: ABSTRACT

Surface outcrops and data from 75 core holes were used to study the Pennsylvanian Kanawha Formation in a 100 mi2 area of western Raleigh County, West Virginia. The interval consists of approximately 1,400 ft of prograding alluvial-deltaic deposits and includes 22 coal seams. The interval developed across drowned orthoquartzite coastal sandstones (Nutall) that mark the approximate top of the New River Formation. Its lower portion (Douglas to Eagle coals) is a generally coarsening-upward, prodelta/distal-deltaic succession of units dominated by dark shale and containing brackish-marine intercalations that extend across most of the area. Coals are widespread but commonly thin (about 1 ft), and the rate of lateral change is low. The top of this lower section is capped by outer deltaic sandstones (Eagle coals). The upper portion of the Kanawha Formation is dominated by outer deltaic strata (to Hernshaw coal) grading upward to inner (Stockton coal) deltaic sandstones. Brackish-marine intercalations associated with autocyclic shifting of detrital lobes are restricted in areal extent. The outer deltaic coal beds are thick (up to about 6 ft) and laterally uniform; succeeding inner deltaic coals are thinner (less than 3-4 ft) and are variable laterally. Deltaic deposits grade upward into lower alluvial plain strata of the Allegheny Formation near the ridge crests. Associated coals locally may be thick (up to 6 ft), but abrupt changes in thickness and shale partings are typical. End_of_Article - Last_Page 1439------------

Productive Continental Block-Fault Deposits of a Mesozoic Back-Arc Basin: ABSTRACT

Continental sediments filling major extensional block-faulted basins in the rock record traditionally are ascribed to plate margin rifting. The Triassic Cuyo basin of central Argentina is such a continental sedimentary basin, which contains over 3,600 m of interbedded fluvial, lacustrine, and volcanic deposits and which produces about 98,000 BOPD. It, however, formed within a back-arc tectonic setting as a reentrant into the Pampean craton prior to the onset of compressional deformation. The southern half of this basin (about 9,000 km2) was studied using logs and cutting descriptions from 101 exploratory wells. The basin began as a series of subparallel fault blocks and accumulated at least 2,750 m of fining-upward detritus in its first cycle of sedimentation. Initial deposits were oxidized lithic conglomeratic units confined by elongated grabens. As faulting waned, high-gradient fluvial sandstones became prevalent (41%), derived primarily from reworking of interbedded felsic tuffs (29%). The end of tensional faulting is marked by widespread bituminous lacustrine shales that form the source beds for oil production. Broad basin subsidence, probably associated with lithospheric cooling, initiated a second episode of basin sedimentation in which tuffs and sediments containing fluvial sandstones (48%) markedly overlap the original basin boundaries. Sandy lag conglomerates of an intermontane brai ed-stream system cap the interval. These deposits were temporarily interrupted by valley basalt flows (Upper Jurassic-Lower Cretaceous), which are similar in composition to diabase dikes of the underlying strata. End_of_Article - Last_Page 1449------------

Early Campanian Coastal Progradational Systems and Their Coal-Forming Environments, Wyoming to New Mexico: ABSTRACT

Ammonite zones (Baculites obtusus-Scaphites hippocrepis) in the marine facies associated with the Mesaverde Formation in the Bighorn basin, Wyoming, Star Point Sandstone and Blackhawk Formation in the Wasatch Plateau, Utah, and the Point Lookout Sandstone, Menefee Formation, and Crevasse Canyon Formation in the Gallup coalfield, New Mexico, indicate that these formations were deposited during early Campanian time (80-84 Ma). The coal-forming environments of these early Campanian formations were located landward of wave-reworked coastal sand complexes of progradational systems along the western margin of the Cretaceous seaway from Wyoming to New Mexico. The Mesaverde coals accumulated in swamps of the lower delta plain and coeval interdeltaic strandplain environments. The Star Point-Blackhawk coals accumulated in swamps of the lower delta plains of laterally shifting, prograding deltas and associated barrier ridge plains. The Point Lookout, Menefee, and Crevasse Canyon coals formed in swamps of the lower delta plain and infilled lagoons behind barrier islands. Although the common coal-forming environments of these progradational systems are back barrier and delta plain, the former setting was the more conducive for accumulation of thick, laterally extensive coals. Economic coal deposits formed in swamps built on abandoned back-barrier platforms that were free of detrital influx and marine influence. Delta-plain coals tend to be lenticular and laterally discontinuous and thus uneconomic. The early Campanian coal-forming coastal-plain environments are analogous to modern peat-forming environments along the coast of Belize, Central America. Deltaic sediments deposited along the Belize coast by short-headed streams are reworked by waves into coastal barrier systems. End_of_Article - Last_Page 855------------

Facies Control of Upper Cretaceous Cleary and Gibson Coal Members near Gallup, New Mexico: ABSTRACT

End_Page 556------------------------------The Upper Cretaceous Point Lookout Sandstone separates the Cleary Coal Member of the Menefee Formation from the Gibson Coal Member of the Crevasse Canyon Formation in much of the Chaco slope and southwest San Juan basin. These coal-bearing members merge as the intervening Point Lookout Sandstone pinches out to the southwest in the direction of the Nutria monocline and Gallup coal field. The deposits of the merged coal interval supported widespread mining activity northwest and north of Gallup, New Mexico, near the turn of the century; however, renewed mining of these coals for the past few years has concentrated northwest of the town. Closely spaced outcrop sections provided data from which to infer the depositional settings of the merged coals. Northwest of Gallup (Enterprise mine area), an alluvial facies of channel sandstones, deposited by northward-flowing streams is complexly interspersed with interfluvial siltstones and with laterally discontinuous coal deposits. North of Gallup (Gibson and Heaton mine areas), better integrated sandstones deposited in north-to-northeast flowing distributary channels, subordinate crevasse-splay sandstones, more widespread coal zones, and common bioturbations in laminated carbonaceous siltstones suggest coal accumulation in a deltaic to lagoonal transition environment. Further northeast of Gallup, where the Nutria monocline changes to an easterly strike toward the Chaco slope, well-developed coals remain associated with the deltaic and lagoonal environments. However, these coal-forming en ironments grade northeastward into the Point Lookout back-barrier to lagoonal transition environments, where coal deposits are poorly developed. End_of_Article - Last_Page 557------------

Variations in Cretaceous Coal-Bearing Strata, Gallup Coal Field, New Mexico: ABSTRACT

Cretaceous coals of the western Gallup field, New Mexico, occur with detrital wedges that interfinger to the northwest with brackish-marine sediments of the San Juan basin. This study documents detailed stratigraphic relations and the relation of coal occurrence to depositional environments of the Gallup, Crevasse Canyon, and Menefee formations. One hundred and fifteen sections form the basis for three-dimensional reconstructions of a 30-sq km area northwest of Gallup. The regressive Gallup Sandstone represents reworking of river-mouth sands into coastal barriers where coals accumulated in back-barrier subfacies. This formation grades upward into the Dilco Coal Member of the Crevasse Canyon Formation characterized by varve-like fine-grained sediments, thin sandstones, and coals, which pass upward into thick, northwesterly transported fluvial sandstones. These merge laterally and upward into northerly oriented, thick paleochannel sandstones of the Bartlett Barren Member of the Crevasse Canyon Formation. The coal-bearing Gibson Coal Member of the Crevasse Canyon Formation, which cannot be differentiated from the Cleary Coal Member of the overlying Menefee Formation, contains finer grained sediments and coals deposited in a broad interfluvial depressio bounded on the west by Bartlett alluvial channel facies. Coal accumulation in this depression was terminated by southeasterly oriented, crevasse-like deposits associated with thick fluvial sandstones (Menefee Formation). Stratigraphic variations of coal beds are directly related to their proximity to contemporaneous channel facies. Uniformly thick coals trend subparallel to channel facies; near the channel facies, coals become erratic and pass into rooted, carbonaceous overbank detritus. Coals are offset locally by faults caused by differential compaction beneath overlying channel sandstones. End_of_Article - Last_Page 689------------