Peloidal Grainstone Research Articles

アブダビ海域におけるＡｒａｂ層の堆積相について

Upper Jurassic Arab Formation consists of evaporite and carbonate sediments deposited in the Arabian platform. From the observation of slubed cores and thin sections of the Arab Formation of 13 wells in offshore, Abu Dhabi, six types of carbonate rock facies have been determined. These are 1) algal mat mudstone and boundstone; 2) bioturbated wackestone and packstone, 3) peloidal bioclastic intraclastic grainstone and packstone, 4) bioclastic wackestone, packstone and boundstone, 5) well sorted oolitic peloidal grainstone, and 6) peloidal bioclastic wackestone and mudstone. Former three of these six rock facies were formed in the tidal flat environment. These are intercalated with anhydrite layers and occasionally dolomitized. Later three formed in the shallow shelf and lagoonal environments. The stratigraphic relationship and distribution of those rock facies in the Arab, Formation show the shallowing upward sequence from the shallow open marine carbonate shelf to supratidal environments.

Upper Jurassic Ramp Carbonate and Associated Evaporite, Neuquen Province, Argentina: ABSTRACT

The Oxfordian La Manga Limestone (10-65 m) and overlying Auquilco Gypsum (315 m maximum thickness) crop out along the west flank of the Neuquen basin, Neuquen Province, Argentina (36/sup 0/40/sup 0/S lat.). The contact with the underlying Lotena Sandstone is gradational, and both formations are cut by the Late Jurassic Araucanian angular unconformity. Seven lithofacies have been identified within sections measured through the entire interval along the northeast to southwest trending, 30-km long Sierra de la Vaca Muerta ridge (38/sup 0/30'-39/sup 0/S). The La Manga Limestone is interpreted as a temperate ramp carbonate that developed over the Lotena Formation siliciclastic shelf. Interpretations of lithofacies from southwest to northeast are: behind-barrier subtidal lagoon with washovers; coral and red algae biostromes; ooid and peloid sand shoals; downslope wackestone and packstone mud mounds; and deep-water carbonate turbidites. A minor regression separates La Manga and Auquilco Formations. Lithofacies of the Auquilco Formation indicate a shallowing-up sequence comprised of initially deep (hundreds of meters) subaqueous evaporite deposition followed by shallow, subtidal carbonate peloidal and shell fragment grainstones and evaporites. Thickness of the subaqueous evaporite gives an order of magnitude estimate of Auquilco basin depths of a few hundred meters at most. The Neuquen basin hasmore » an intermediate proportion of carbonate in comparison to relatively carbonate-poor basins to the south and carbonate-rich basins to the north.« less

Model for Sandstone-Carbonate Cyclothems"Based on Upper Member of Morgan Formation (Middle Pennsylvanian) of Northern Utah and Colorado: DISCUSSION"

The upper member of the 200 m (660 ft) thick Morgan Formation (Middle Pennsylvanian) consists of 5-25 m (16-82 ft) thick, very fine-grained quartz sandstone units that are interbedded repetitively with 0.5-11 m (1.6-36 ft) thick, oolitic, bioclastic, peloidal, and micritic carbonate units. A modified Markov chain analysis verified the presence of 2 kinds of repetitive, regressive-dominant, shoaling-upward vertical sequences. The carbonate sequence consists of, from bottom to top: (a) a knifesharp contact between sandstone and the overlying carbonate unit, (b) planar-bedded bioclastic grainstones or cross-bedded oolitic grainstones, (c) fragmented-fossil and intact-fossil wackestones, (d) peloidal and evaporitic(?) wackestones and mudstones, (e) peloidal and ostracod-gastropod grainstones and packstones, (f) planar-bedded to low-angle cross-bedded fossiliferous calcareous sandstones, commonly comprising the transitional contact between carbonate and overlying sandstone. The sandstone sequence consists of, from bottom to top: (f) planar-bedded to low-angle cross-bedded fossiliferous calcareous sandstone, (g) medium-scale trough and wedge cross-bedded sandstones punctuated by bioturbated zones dominated by Ophiomorpha nodosa, (h) large to very large-scale wedge and planar-tabular cross-bedded sandstones, (i) deformed cross-bedded sandstone erosionally truncated by the next overlying carbonate unit. Both sequences define a new conceptual depositional model that may explain the origin of many other widespread interbedded quartz sandstone-carbonate successions deposited on the western edge of the North American craton during Pennsylvanian and Permian time. One depositional phase employs virtually instantaneous transgression, followed by dominantly progradational subtidal carbonate-shelf to shoal conditions. The other involves progradationof sandstones across intertidal and subtidal carbonate environments. Eustatic changes related to Gondwana glaciation were probably the primary control on sedimentation. The quartz sandstone-carbonate "cyclothems" defined by this study have potential as targets for hydrocarbon exploration. Both eolian dune sandstones and dolomitized shelf carbonate strata are locally important reservoir rocks in the subsurface in parts of the Western Overthrust Belt in Utah and Wyoming.--Modified journal abstract.

Platform and off‐platform carbonates of the Upper Cambrian of western Maryland, U.S.A.

ABSTRACTUpper Cambrian carbonates in western Maryland are comprised of platform facies (Conococheague Limestone) west of South Mountain and basin facies (Frederick Limestone) east of South Mountain. Conocheague platform carbonates contain interbedded non‐cyclic and cyclic facies. Non‐cyclic facies consist of cross‐stratified grainstones, thrombolitic bioherms, and graded, thin‐bedded dolostones. These were deposited in shallow, subtidal shelf lagoons. Cyclic facies are composed of repeated sequences of cross‐stratified grainstone; ribbon‐rock; wavy, prism‐cracked laminite; and planar laminated dolostone. The cyclic facies are shallowing‐upward cycles produced by lateral progradation of tidal flats over shallow, nearshore subtidal environments. Cyclic and non‐cyclic facies are interbedded in the Conococheague in a layer cake fashion, but no higher‐order cyclicity can be found.The Frederick Limestone is dominated by monotonously thick sequences of graded, thin‐bedded limestones, interbedded with massive peloidal grainstones and beds of breccia up to 10 m thick in the lower Frederick. The breccias contain transported megaclasts of Epiphyton‐Girvanella boundstones. The basal Frederick was deposited in a slope‐to‐basinal setting east of a rimmed shelf. An Epiphyton‐Girvanella marginal reef along the shelf edge was the source of the blocks in the breccias. The upper Frederick Limestone formed on a carbonate ramp.

Compactional Features in Cambro-Ordovician Carbonates of Central Appalachians and Their Significance: ABSTRACT

Compactional features are well known from siliciclastic rocks, but it is generally believed that carbonates have undergone little burial compaction. However, in the 3.5-km thick Cambrian-Ordovician carbonates of the Central Appalachians, many small-scale compaction features have been recognized. Evidence for differential compaction is: (1) wrapping of thin beds around meter-scale early-cemented algal bioherms and (2) sedimentary boudinage and pinch-and-swell features in interlayered thin beds of carbonate grainstone and mudstone. The grainstone layers have deformed in a brittle manner (cracking or yielding boudins), whereas mudstone layers behave ductily (flowing and bending around boudins), indicating that at the time of burial, grainstone layers were lithified but mudst ne layers were unlithified. Burrows and shells in sandy layers are preserved, but burrows are deformed and shells broken in muddy layers. Pervasive, rather than differential, compaction in muddy carbonates is evidence by flattened burrows, rotation of platy allochems parallel with End_Page 257------------------------------ bedding, telescoping of mudcracks, and anastomosing wispy argillaceous seams. Pervasive compaction in peloidal grainstone is evidenced by warping and cracking of internal layers that are outlined by wispy seams. Ooid and skeletal grainstones and algal bioherms do not show these compaction features. Time lines in these Cambrian-Ordovician carbonates converge across depositional strike from east to west, and this coincides with a change in facies from shelf-margin algal bioherms and grainstones showing little compaction to lagoon-peritidal mudstones with abundant compaction features. Volume reduction by compaction is clearly facies controlled and also has influenced the geometry of the time lines. End_of_Article - Last_Page 258------------

Controls on Porosity and Dolomite Distribution in Upper Smackover Formation (Upper Jurassic), Southwestern Alabama and Western Florida: ABSTRACT

Upper Smackover carbonates of southwestern Alabama and western Florida exhibit arcuate trends of porous dolomitized grainstones separated by areas of impermeable muddy limestones. The origin of these porous trends is related to the depositional and diagenetic history of upper Smackover carbonates and overlying Buckner evaporites. Shoal-water oolitic and peloidal grainstones of the upper Smackover were deposited across basement and salt-related topographic highs. Subsequent aggradation and stepwise progradation of oolitic shoals over low-energy packstones and wackestones produced a complex sea-floor topography of arcuate oolite ridges (highs) and elliptical muddy lagoons (lows). Marine regression during Buckner deposition led to the formation of saline ponds and sabkhas th t were initially located over Smackover lagoonal lows and rimmed by Smackover oolite ridges. Precipitation of evaporites within these depressions created magnesium-rich brines that selectively dolomitized adjacent Smackover carbonates. Both the outflow of brine and the pattern of dolomitization were controlled by the fluid transmissibility of Smackover sediments. Consequently, permeable oolitic and peloidal grainstones were preferentially dolomitized over less-permeable muddy carbonates. A possible hydrologic analog for the study area exists within the MacLeod Evaporite basin, western Australia. Following burial, dolomites maintained greater effective porosity on the flanks of basement and salt-related topographic highs than on the crests. End_of_Article - Last_Page 274------------

Neptunian Dikes and Sills in Devonian Reef Complexes of Canning Basin, Western Australia: ABSTRACT

Neptunian dikes and sills are conspicuous features of early-cemented limestones in Devonian reef complexes of the Canning basin. They are sedimentary fillings of fissures formed by fracturing of the limestones soon after deposition, and are most abundant in the reef subfacies, but also occur in some back-reef and marginal-slope deposits. The dikes and sills were filled with contemporary sediment (including oolite, peloid grainstone, biomicrite, and calcareous sandstone), encrusting organisms (Renalcis and stromatolites), and early radiaxial calcite cement. Some of the sediment is dolomitized or mineralized. Some dikes contain concentrically coated balls of radiaxial calcite up to 0.3 m (1 ft) across. Sediment in the dikes and sills was cemented rapidly, and repeated refracturing and infilling occurred during brief intervals. The largest dikes are up to 6 km (3.5 mi) long, 20 m (65 ft) wide, and originally extended more than 80 m (260 ft) below the surface. However, most are less than 100 m (330 ft) long and 5 cm (2 in.) wide. Neptunian sills are up to 0.8 m (2.5 ft) thick. The principal strike directions of major dike approximately parallel the original reef fronts of underlying basement ridges. Lateral extension through the development of fissures amounted to as much as 6.5%, and averaged about 3% in measured reef sections. The fissures allowed circulation of large volumes of seawater to considerable depth within the reef complexes, and must have played a significant role in early diagenesis. Fissuring probably resulted from earthquake shaking, differential compaction of underlying sediments, and slippage along bedding planes, especially in the steeply dipping marginal-slope deposits. End_of_Article - Last_Page 517------------

Cyclic Sedimentation in Middle and Upper Holder (Upper Pennsylvanian) Formation, New Mexico: ABSTRACT

The Middle and Upper Holder Formation (Upper Pennsylvanian), Sacramento Mountains, New Mexico, comprises terrigenous and carbonate facies that accumulated on a narrow open-marine to restricted-marine shelf. Differential subsidence of the shelf combined with eustatic sea level changes caused cyclic deposition. Deposition began with a varying supply of terrigenous sediments from shifting delta lobes. Several carbonate facies, lateral equivalents of the terrigenous strata, are distinguished on the basis of their allochemical constituents. Bioclastic wackestones to grainstones with cortoids and oncoids characterize open-marine intervals. Laminoid-fenestral mudstones and wackestones with oncoids were common in restricted-marine intervals. Fusulinids and algal and peloidal grainstones are common marginal-marine facies. Restricted-marine sediments were deposited after clastic influxes and during marine transgressions. The division between the Middle and Upper Holder members can be placed beneath a channel conglomerate an unconformity that developed on a restricted platform. Deposition of predominantly restricted-marine sediments with erosional clastic influxes characterize the Upper Holder. The upward increase both in reworked and encrusted allochems might End_Page 541------------------------------ reflect regional changes in paleogeography that caused progressive exposure of the platform. Rates of sea level rise were high and punctuated by periods of decreased rates or near stillstand as indicated by restricted-marine and clastic deposition with unconformities. The duration of these stable periods of relative sea level increased through time. End_of_Article - Last_Page 542------------

Depositional History of Smackover Carbonates in Manila Embayment of South Alabama: ABSTRACT

Upper Jurassic Smackover carbonate deposition in south Alabama was significantly influenced by a number of paleotopographic highs which divided the region into several subbasins or embayments. The Manila embayment in Monroe, Conecuh, and Clarke Counties was bounded to the south by the Conecuh ridge and the Wiggins arch and to the west by diapiric salt ridges. These paleotopographic highs protected the interior of the embayment from wave and current energy and significantly influenced Smackover deposition. Initial carbonate deposition, represented by intertidal oncolitic and peloidal packstones and algally laminated mudstones, occurred in the deeper portions of the embayment, and was accompanied by the continuation of subaerial clastic deposition on the still-emergent highs. With sea level rise, these intertidal deposits graded upward into subtidal peloidal packstones containing sparse faunal assemblages indicative of the restricted circulation within the embayment. By middle Oxfordian time, sea level rise had inundated the paleohighs, eliminating restrictive conditions in the embayment. Deposition in the embayment is represented by peloidal, oolitic, and oncolitic packstones and grainstones containing open marine faunal assemblages, while the paleohighs became the site of oolitic shoal with local patch-reef development. A decrease in the rate of sea level rise during late Oxfordian time allowed the carbonate deposits to build toward sea level. The oolitic shoal deposits on the paleohighs pass upward into evaporitic sabkha deposits, whereas the deposits of the embayment grade into oolitic shoal and intertidal flat deposits and finally into sabkha and evaporitic lagoon deposits. End_of_Article - Last_Page 468------------

Storm Deposits (Tempestites) in Ordovician Cratonic Carbonates (Arbuckle Group, South-Central Oklahoma): ABSTRACT

The Early Ordovician Kindblade Formation (Arbuckle Group), exposed in the Arbuckle Mountains of south-central Oklahoma, is a shallow marine epicontinental carbonate sequence that contains numerous storm deposits. Similar deposits also occur in other Arbukle Group units, although not as abundantly as in the Kindblade Formation. The storm deposits (tempestites) are of two types, proximal and distal; the latter dominates in terms of both number and aggregate thickness. Distal tempestites consist of a fining upward sequence, 5 to 50 cm (2 to 20 in.) thick, that overlies an eroded hardground or firmground. The sequence consists of a lag lithoclastic grainstone that grades up into a laminated peloidal grainstone and then into mudstone. Firmgrounds are characterized by hummocky, sharp, and erosional contacts (relief 2 to 7 cm, 0.75 to 2.75 in.) with grainstone-filled erosive pockets. Hardgrounds are characterized by sharp hummocky-to-convolute surfaces (relief < 4 cm, 1.5 in.), which are mineralized and bored. Primary sedimentary features such as laminations, burrows, and allochems are truncated at the surfaces, and borings are filled with unsorted lithoclasts. The lithoclasts at the base of the sequence are bored, generally well rounded, discoid in shape, and consist of mudstone, peloidal packstone, and oolitic grainstone. Infiltration fabrics within the lithoclastic grainstone include cement-filled shelter voids beneath large clasts and internal sediment perched on the upper surfaces of lithoclasts. The overlying peloidal grainstones contain ripple cross-la inations, plane-laminations, and hummocky cross-stratification as well as rare escape burrows. The overlying mudstone is sparsely fossiliferous and bioturbated with burrows either selectively dolomitized or infilled with lithoclastic grainstone. Although there are many examples of the ideal sequence described above, complex composite or amalgamated beds are also common. Proximal tempestites consist of coarse lithoclastic flat pebble conglomerate beds approximately 1 m (3.25 ft) thick that are interbedded with ooid grainstone and overlie mudstone. The contact between the units is sharp and erosional. The lithoclasts are of variable composition and may be up to 20 cm (7.75 in.) in diameter. The two types of tempestites occur in crude cycles, which consist of distal deposits overlain by proximal tempestites and ooid grainstones. The cycles are interpreted as shallowing-upward progradational sequences. The abundance of the storm deposits in the section, approximately one every 20 cm (7.75 in.), indicates that hundreds of storm-induced events are recorded in the Kindblade Formation. The tempestites represent rare catastrophic events, while the hardgrounds-firmgrounds are discontinuity surfaces that represent gaps in the sequence. End_of_Article - Last_Page 471------------

Corals of Madison Group (Mississippian), Williston Basin, North Dakota: ABSTRACT

Coral faunas studied from subsurface cores of the Mississippian Madison Group in the Williston basin of North Dakota indicate that Sando's coral zones for outcrops in western North America can be extended into the subsurface of North Dakota. Coral zones II and III are recognized as corresponding roughly to lower and upper Mission Canyon strata, respectively. These data were obtained from 12 wells along the northern border of North Dakota in Divide, Burke, Renville, and Bottineau Counties, and two wells near the center of Williston basin in Dunn and McKenzie Counties. Coral faunas appear to show a relatively low diversity of 13 species distributed among the following genera: Syringopora, Vesiculophyllum, Sychnoelasma, Amplexizaphrentis, Lophophyllum, Cyathaxonia, Lithostrotion (Siphonodendron), Diphyphyllum, Michelinia, and Stelechophyllum. Vesiculophyllum, Sychnoelasma, and Syringopora are the most abundant genera of the Madison Group in North Dakota. Corals found in dark argillaceous crinoid-skeletal wackestones representing deeper waters are robust, and this may infer a hospitable environment for their growth. However, evidence from the coral and lithologic associations refute the pervading dogma that the occurrence of corals is strictly facies controlled. Abundant smaller corals have been found from buff-colored skeletal wackestones and algal mudstones which alternate with subaqueous anhydrites representing a marginal marine environment. In addition, corals have been found in buff-colored skeletal and peloidal grainstones of adjacent shoals and in brown pisolitic-oolitic packstones-wackestones of possible tidal ponds. These latter deposits may represent allochthonous accumulations, but the amount of time involved in transport f corals would not invalidate their usefulness as biostratigraphic tools. End_of_Article - Last_Page 1360------------

Wavy and Lenticular-Bedded Carbonate Ribbon Rocks of the Upper Cambrian Conococheague Limestone, Central Appalachians

ABSTRACT Thinly interbedded limestones and dolostones are common in Paleozoic and Proterozoic carbonate sequences. Such rocks, called ribbon rocks, are characteristic of the 750-m-thick Upper Cambrian Conococheague limestone, a platform deposit exposed in western Maryland. Limestones of Conococheague ribbon rocks occur as continuous wavy thin beds to isolated centimeter-scale pods; commonly have flat bottoms and ripple-form tops; typically are internally current- or wave-ripple cross-laminated; and are composed of peloidal grainstones. Dolostones interbedded with these limestones are typically continuous on the outcrop scale; drape underlying limestone ripple-forms; are commonly mudcracked; and are composed of 10-40 B. dolomite rhombohedrons in a carbonate-clay matrix. Some ribbon rocks are di rupted by filled tubes, soft sediment deformation features, and abrupt lateral changes in rock type. Ribbon rocks within meter-scale cycles of the Conococheague have gradational contacts with cross-stratified grainstones below and grade into mud-cracked laminites above. These observations suggest that the limestone-dolostone alternations of ribbon rocks reflect original carbonate sand-mud alternations. This interpretation is strongly amplified by comparing Conococheague ribbon rocks to examples of lenticular and wavy bedding from mixed siliciclastic sand and mud flats from the modern North Sea. Conococheague ribbon rocks are interpreted as ancient carbonate examples of lenticular and wavy bedding deposited on mixed carbonate sand and mudflats in a shallow subtidal to intertidal setting. This is consistent with an overall interpretation of Conococheague cycles as the regressive deposits of carbonate tidal flats.

Stratigraphy and Sedimentology of Upper Helderberg Group (Lower Devonian), New York: ABSTRACT

Field and petrologic study of the upper Helderberg Group has elucidated facies relationships. A tongue of the Kalkberg formation, recognized locally by Rickard (1962), at the top of the lower Helderberg cycle, extends southeastward to Catskill Creek where it thickens slightly, changes lithologic character, and passes laterally into the basal Becraft formation of the upper cycle. The Becraft represents a progradational phase within the general Lower Devonian transgression, and is seen as correlative with parts of both cycles. Thus, sedimentation was continuous between the two transgressive cycles. These revisions only slightly alter lower-cycle stratigraphy and preserve the integrity of Rickard's time lines. The Becraft formation is divisible into three units. (1) (Base) High-angle, cross-stratified, crinoidal grainstones interbedded with shale, cross-laminated, silty, peloidal grainstones, and packstones. (2) Trough cross-stratified, gypidulid-bearing, crinoidal grainstones with rare channels. (3) (Top) Finer grained, bioturbated to vaguely laminated grainstones containing gypidulids, and in-situ, rootlike crinoid holdfasts, but less crinoidal debris. Units 1 and 2 represent progradation of a tidally influenced, skeletal-sand shoal over shoal-margin and deeper subtidal environments. Unit 3 is transitional to the superjacent Alsen formation. The Alsen formation consists of irregularly bedded, bioturbated, skeletal, and peloidal grainstones. Bryozoans, brachiopods, and End_Page 1167------------------------------ micritized skeletal debris are the dominant particles. Peloids are abundant in the middle of the formation. The Alsen represents slightly deeper water and less current activity than the equivalent upper Becraft. Port Ewen lithologies include irregular and nodular, silty, skeletal wackestones and packstones, silty, peloidal packstones, and interbedded shale. Ostracodes, bryozoans, brachiopods, and micritized skeletal grains represent the sparse fauna. Bioturbation is ubiquitous; Chondrites, Planolites, and Zoophycos lebensspuren are common. The Port Ewen formation records the deepest, least-oxygenated environment in the Helderberg Group. End_of_Article - Last_Page 1168------------

Lithofacies and Depositional Environments of Middle Ordovician Shelf Carbonates in Southern Appalachians: ABSTRACT

Detailed field and laboratory observations are the basis for the description of 13 lithofacies in the Middle Ordovician Chickamauga Group in Jefferson County, Alabama. These lithofacies, including stromatolitic, fenestral and bioturbated mudstones, and varieties of skeletal and peloidal wackestones, packstones and grainstones, represent deposition in seven major environments: high intertidal, low intertidal, tidal channel, subtidal wave baffle, subtidal bar, subtidal bank, and subtidal level bottom. Middle Ordovician deposition began on the extremely irregular karstic surface of the Cambro-Ordovician Knox Dolomite. A lag deposit of reworked dolomite and chert clasts (Attalla Member) is overlain by intertidal deposits. High and low intertidal lithologies vary laterally in hickness in response to deposition over highs present on the irregular Knox surface. With continued transgression, shallow-water facies were succeeded by a suite of subtidal deposits composed dominantly of level-bottom lithologies deposited below wave base. Changes in sedimentation and/or subsidence led to a gradual shallowing and deposition above wave base. Coarse-grained, high-energy tidal bars and skeletal bank deposits dominate this part of the sequence. In the southern part of the study area, subtidal level-bottom lithologies dominate the upper part of the Middle Ordovician sequence. To the north, these lithologies grade laterally into shallower subtidal and intertidal lithologies. The sequence is unconformably overlain by the Upper Ordovician Sequatchie Formation. End_of_Article - Last_Page 1433------------

Early Carbonate Fabrics in Silurian Reefs of Gotland, Sweden: ABSTRACT

The Hogklint reefs of Gotland occur within a shallowing-upward sequence and are developed in the outer zones of a carbonate wedge adjacent to a shale belt. Shoal and peritidal sequences are the major carbonates which occur in the inner parts of the wedge and are preferentially associated with erosion surfaces. The Hogklint reefs show carbonate fabrics that are inferred to indicate synsedimentary marine cementation of the reefs. These fabrics are: (1) pseudofibrous calcites that are believed to have replaced acicular/fibrous cements with original aragonite and/or Mg-calcite mineralogy, (2) pelmicsparite-pelmicrite areas, (3) stromatolite-like crusts, and (4) early micritic cements. The reefs also show evidence of subaerial exposure by the occurrence of erosion surfaces at the top of the reefs, dissolution cavities, and vadose crystal silt infills in cavities in the algal biofacies which cap the reefs. End_Page 640------------------------------ The distribution of the various cement types within the reefs can be linked with the shallowing-upward nature of the limestone sequence as there is a predominance of early marine cement fabrics in the middle and upper parts of the reefs, particularly in the algal biofacies. The spatial and sequential nature of the various carbonate diagenetic fabrics show that the Hogklint reefs went through four main diagenetic events. Microstalactitic cements, vadose crystal silts, grain-contact cements, and pseudofibrous isopachous cements are developed within cross-bedded peloidal grainstones at the top of the shallowing-upward sequence and indicate cementation within the freshwater vadose zone and intertidal and/or marine phreatic zone. End_of_Article - Last_Page 641------------

Influence of Early Cementation on Dolomitization and Compaction in Cambro-Ordovician Carbonates of Central Appalachians: ABSTRACT

The Cambro-Ordovician platform carbonates of the Great Valley, central Appalachians, are notable for the interlayering of limestones and dolostones with well-preserved primary sedimentary structures. Despite such primary structures there are several observations that demonstrate a secondary origin for much of the dolomite, the most significant being that dolostone-limestone boundaries commonly crosscut bedding planes. The nature of such dolomitization is clearly revealed by certain mottled dolostones in which mudcracks and burrows are haloed by limestone. Stratification can be traced through the 2 to 10-mm wide limestone halos into the dolostone without break. The limestone halos have a peloidal grainstone texture with high minus-cement porosity and unbroken shells wherea the dolostone is a sucrosic mudstone with fragmented shells. We interpret these observations to mean that early cementation around permeable mudcracks and burrows has protected these zones from later compaction and dolomitization. We extend this idea of selective early cementation of permeable sediments to explain why the limestones are invariably grainstones while the dolostones are mudrocks. The idea is supported by the observation that laminae of dolostones are bent around limestone mudcrack halos, ripple lenses, etc, showing that the limestones have acted competently and the dolostones incompetently during compaction. The timing of this post-early cementation dolomitization is not unequivocal, but it is certainly pre-stylolitization, as demonstrated by dissolution of dolomite rhombs along bedding-plane stylolites. Our observations do not rule out the possibility of a syndepositional origin for at least some dolomite in these rocks (there are dolomites and dolomites!) but they do negate the pressure-solution origin of dolomite advanced by Wanless (1979). End_of_Article - Last_Page 577------------

Paleoecology and Structure of a Late Silurian-Early Devonian(?) Patch Reef, Northwestern New Jersey: ABSTRACT

Quantitative and qualitative field examination of an Upper Silurian-Lower Devonian(?) bioherm in northwestern New Jersey resulted in the identification of at least five distinct lithologic and biotic facies: (1) a basinal lithofacies of medium to thick-bedded pelagic limestones containing few discernable fossils; (2) a peloidal grainstone facies, which is dominated by pelmatozoan fragments and peloidal carbonate grains; (3) a reef-talus facies characterized by a high biotic diversity; (4) a coral-boundstone facies dominated by massive tabulate corals; and (5) a stromatoporoid reef facies composed of large juxtaposed hemispherical stromatoporoids (65%), tabulate corals (30%), and rugose corals (5%). Facies evidence demonstrates that the stromatoporoids and corals organically cemented this buildup into a rigid, wave-resistant structure that controlled its surrounding environment. According to Heckel's 1974 classification of reefs, this type of boundstone lithofacies would indicate an organic framework reef. The patch reef grew in a moderate-to-high energy regime and was developed on a carbonate platform which was situated on the margin of an epeiric sea. Reef growth was progradational and upward in response to a relative rise in sea level. Investigation of the patch reef facies revealed compound growth and accretion zones, in addition to demonstrating distributional patterns similar to those of modern patch reef analogs. End_of_Article - Last_Page 1173------------

Pennsylvanian mud mound, Hermosa group, Southeastern Colorado

A mixed organic-hydrodynamic origin Is suggested for a carbonate mud mound In the Pennsylvanian Hermosa Group, San Juan Mountains, Colorado. The lower portion of the mound consists of fossiliferous wackestone composed of phylloid algae, echinoderm plates, foraminifera and other subordinate fossil types. A systematic increase in the percentage of phylloid algae toward the center of the mound suggests that the mound originated as algal bank. The algal mound differs from more abundant phylloid algal mounds in the Hermosa Group to the southwest of the study area by being smaller in size, by lacking spar cement, and by having a smaller percentage of algae in a matrix-supported texture. The mound eventually grew into a hydrodynamic barrier. Currents of varying strength transported and deposited peloidal packstone, peloidal grainstone, and peloidal-intraclastic-oolitic grainstone on top of the mound. Breccla zones at the crest of the mound, interpreted to be the result of subaerial exposure, indicate that deposition ceased as growth of the mound reached sea level.

Submarine Carbonate Cementation and Pisolith Growth in Silurian Reefs of Northern Indiana: ABSTRACT

Peloidal grainstone and pisolite fill crevices on the flanks of several Silurian reefs in northern Indiana, producing intersecting dikes and veins. Internal dike structures and cross-cutting show that the cycle of crevice formation, sediment trapping, and lithification was repeated several times during the life of the reefs. Significantly, where younger dikes cut across older dike material, their walls have sharp boundaries and matching wall structures, showing that each earlier crevice sediment became lithified before the next fracturing event occurred. The grainstone clasts appear to have been mechanically swept into the crevices and initially lithified by the precipitation of sparry calcite cement around the grains. Most of the crevice fill now consists of calcite or dolomite pisoliths, however, some exhibit concentric structure, suggesting concretionary growth. Other pisoliths exhibit radial structure in which blades of cloudy calcite encroach on peloids and fossil fragments of the grainstones, suggesting growth by replacement. Petrographic evidence thus favors in-situ growth of the pisoliths, and suggests that lithification of the grainstone was completed by this process. Evidence favoring vadose cementation of the grainstone, or favoring vadose origin of the pisoliths, is absent, and fresh-water phreatic cementation is considered unlikely. However, the repeated alternation of cementation events with marine reef sediment-producing conditions strongly suggests that initial grainstone cementation and pisolith growth occurred under shallow-water marine conditions. End_of_Article - Last_Page 790------------

Carbonate Facies Variation on Guadalupian Shelf Crest (Upper Yates and Lower Tansill Formations), Guadalupe Mountains, New Mexico: ABSTRACT

End_Page 501------------------------------The Guadalupian shelf crest provides a unique setting for the study of changing carbonate-shelf environments. Previous works by King, Newell, Dunham, Smith, Pray, and others have provided models and interpretations to define a generalized morphology of the carbonate facies for the Guadalupian shelf, behind the Capitan Limestone. Detailed field studies of about 50 m of carbonate units within the upper Yates and lower Tansill Formations reveal significantly different facies patterns and suggest changing shelf profiles and environments through time. Alternating carbonate and sandstone units are apparent as the product of cyclic shelf sedimentation. Major carbonate units within the stratigraphic interval studied are informally named (Hairpin dolomite, Triplet dolomite, and basal Tansill dolomite). Recognition of five major carbonate lithofacies within the three identified carbonate units at the shelf crest (the paleotopographic high of Dunham's marginal mound) indicates variance in deposition owing to highly restricted water conditions as well as more normal marine water circulation. The three carbonate units differ in the following respects: the Hairpin dolomite is dominated by fenestral peloid grain-supported, pisolite-rich facies, and is associated with erosion surfaces and abundant tepee structures. Carbonate facies in the Hairpin dolomite repeatedly grew to fill level, and the shelf crest extended at least 3 km perpendicular to the Capitan Limestone. The Triplet dolomite is composed primarily of peloidal grainstones with abundant admixed siliciclastic grains. Shelf-crest features, that is, pisolites, tepees, and fenestral strata, are uncommon, suggesting an absence of a nearly emergent paleotopographic high and a more continuous basinward sloping shelf. The basal Tansill dolomite contains abundant skeletal grains, that is, dasycladacean algae, ostracods, ga tropods, and foraminifers, suggesting a deeper shelfward penetration of more open marine water than for the other two units. Erosion surfaces common at the upper boundaries of the identified major carbonate units are most common and traceable horizontally 0.5 to 3 km behind the Capitan front, locally 50 m behind the Capitan Limestone. Simple progradation of the shelf-crest facies with the Capitan Limestone through time does not explain the change of facies within the three major carbonate units. End_of_Article - Last_Page 502------------