Carbonate Terranes Research Articles

Vegetation and cloud cover shape semi‐arid carbonate landform development

AbstractWe study the relationships among precipitation, vegetation, and morphological characteristics of watersheds draining either side of the Dhofar Mountains in southeastern Oman to understand the geomorphic signature of water availability in a semi‐arid carbonate landscape. Water availability is expressed in terms of vegetation and cloud cover. The integral and the statistical moments of the hypsometric curve were used to determine whether hyper‐arid, inland‐draining watersheds are significantly different from seasonally wet watersheds on the coast side of the mountain range. We demonstrate that the vegetation and cloud cover are correlated, with locations with longer cloud periods also having a longer period with a vegetation canopy. The analysis shows that the hypsometric curve and its statistical moments capture the morphological difference between wet watersheds shaped by groundwater sapping and dry watersheds with fluvial morphology. Specifically, the curves exhibit two shapes: watersheds with more vegetation and cloud cover are characterized by higher convexity, and those with less vegetation and cloud cover are characterized by higher concavity. A variance analysis of cloud cover, vegetation, and hypsometric integral shows that they are significantly different between the wet and dry watersheds. The link between hydrology and morphology is not strong at the scale of a single watershed, but it is significant when the watersheds are aggregated in zones. The statistical moments of the hypsometric curve in the range of values of the integral and skewness show good separation between watersheds dominated by sapping and fluvial erosion processes. We can separate the watersheds draining the mountain range in two distinct groups on the basis of their bimodal hydrological and morphological characteristics. Our findings support other studies that hypothesize a trade‐off from chemical‐ to mechanical‐dominated denudation in carbonate terranes as precipitation decreases.

Nature and origin of the nonsulfide zinc deposits in the Sierra Mojada District, Coahuila, Mexico: constraints from regional geology, petrography, and isotope analyses

The Sierra Mojada District comprises multiple types of near-surface mineral concentrations ranging from polymetallic sulfide zones, “nonsulfide Zn” (NSZ) deposits, and a silver-rich Pb carbonate deposit hosted by lower Cretaceous carbonate strata. Hypogene concentrations of Fe-Zn-Pb-Cu-Ag sulfides and sulfosalts are locally preserved and are associated with hydrothermal dolomite and silica. Alteration mineralogy and sulfur isotope data suggest primary Zn-Pb-Ag mineralization from circa 200 °C hydrothermal fluids. The NSZ deposits dominantly consist of smithsonite and hemimorphite associated with local Mn-Fe oxides. The Red Zinc Zone consists of strata-bound zones dominantly of hemimorphite that fills pores in residual and resedimented Fe oxides. The White Zinc Zone shows local dissolution features, including internal sediments interbanded with and cemented by smithsonite. Similar Pb isotopic compositions of smithsonite, hemimorphite, and cerussite to Sierra Mojada galena document that the NSZ deposits originated from polymetallic carbonate-replacement sulfide deposits, with flow of metal-bearing groundwater being controlled by local topography and structural features in this extensional terrane. Oxygen isotope values for Sierra Mojada smithsonite are relatively constant (δ18OVSMOW = 20.9 to 23.3‰) but are unusually low compared to other supergene smithsonites. Using δ18OVSMOW (− 8‰) of modern groundwater at nearby Cuatrocienegas, smithsonite formational temperatures are calculated to have been between 26 to 35 °C. Smithsonite precipitation was favored by near-neutral conditions typical of carbonate terranes, whereas hemimorphite precipitated by reaction with wallrock silica and locally, or episodically, more acidic conditions resulting from sulfide oxidation. Transition to, and stabilization of, the modern desert climate over the past 9000 years from the Late Pleistocene wetter, cooler climate of northern Mexico resulted in episodic drawdown of the water table and termination of local supergene metal mobilization.

U–Pb Dating of Cave Spar: A New Shallow Crust Landscape Evolution Tool

AbstractIn carbonate terranes, rocks types that provide apatite are not available to effectively use apatite fission track (AFT) or (U/Th)‐He chronometry (AHe). Here we suggest that calcite cave spar can be an effective chronometer and complimentary to AFT and AHe thermochronometers in carbonate regions such as our study area, the Guadalupe Mountains of southeastern New Mexico, and west Texas. Our measured depth of cave spar deposition is 500 ± 250 m beneath the regional water table, formed at temperatures of 40° to 80°C, indicating that these caves and their spar crystals form near the supercritical CO2‐subcritical CO2 boundary where we interpret the origin of both the caves and spar to occur. This depth‐temperature relationship suggests a higher than normal geotherm, likely associated with regional magmatic activity. As a case study we examined the timing of uplift of the Guadalupe Mountains previously attributed to the compressional Laramide orogeny (ca. 90 to 50 Ma), later extensional tectonics associated with Basin and Range (ca. 36 to 28 Ma) or the opening of the Rio Grande Rift (ca. 20 Ma to Present). We show that most of the spar origin is coeval with the ignimbrite flare‐up between 36 and 28 Ma. Our results constrain the initiation of Guadalupe Mountains block uplift, relative to the surrounding terrain, to between 27 and 16 Ma and reconstruct the evolution of a low‐lying regional landscape prior to block uplift from 185 to 28 Ma, in support of models that attribute regional surface uplift to extensional tectonics and associated volcanism.

Long-Term Geological Challenges of Dam Construction in a Carbonate Terrane

Many carbonate foundations of dams are afflicted with zones of broken or fractured rock and karstification prior to construction. Rock fractures develop because of tectonic stresses but also because of the removal of overburden loads imposed upon the rock. This is the case with valley stress relief. As fractures control many of earth9s dynamic processes, including fluid transport in the crust, an understanding of valley stress relief is essential for an understanding of karstification. Because water impounded by a structure imposes an increased hydraulic gradient, and because this gradient facilitates the solubility of that carbonate foundation, fractures or bedding planes undergo accelerated transformation by the karstification process and enhance additional dynamics affecting the dissolution rate of carbonates. In a state of nature, karstification requires a time scale of 10,000 to more than 100,000 years. However, when subject to the imposed-increased hydraulic gradients associated with impoundment, the rate of karstification may accelerate a thousand-fold. Civil engineers and geologists have noted that grout curtains or concrete cut-off walls, structures typically used to deal with this problem, offer mixed success in terms of extending the design life of a water retention structure. Traditional repairs impose a new hydraulic gradient causing increased infiltration of the minute, non-groutable, clay-filled microfractures. Construction is an incremental process. The imposed hydraulic gradient increases incrementally as ungrouted microfractures become susceptible to karstification, accelerating development of new seepage channels. Eventually, seepage becomes intolerable as karstification renders the impoundment susceptible to another round of repairs. Four case histories illustrate these concepts.

Cretaceous Tertiary phenomena in the context of seafloor rearrangements and P(CO 2) fluctuations over the past 100 m.y.

Both the bolide impact hypothesis and the volcanism hypothesis suggest, as one of the major environmental consequences, the release of large amounts of SO 2 and CO 2 into the atmosphere, with consequent lowering of the pH of ocean water. In the study of rare earth elements (REEs) in seawater and in carbonate sediments, we found that the Ce in seawater is depleted relative to other REEs due to the partial oxidation of Ce 3+ to Ce 4+ by dissolved oxygen. This oxidation is enhanced by the formation of highly insoluble Ce(OH) 4 and its removal from seawater. The relative Ce depletion is expressed as the Ce anomaly, Ce A*. A quantitative expression for relating Ce A* with pH and P O 2 has been derived. Owing to the involvement of OH − in this process, Ce A* is essentially controlled by the pH of seawater. The REE pattern in seawater is preserved in carbonate sediments. Therefore, the Ce anomalies in marine carbonate sediments provide a unique tool for recording pH changes in paleo-ocean water. Furthermore, the pH of ocean water is controlled by the partial pressure of CO 2, P CO 2 , in the atmosphere; therefore, the corresponding P CO 2 changes are derived. About 340 Pacific carbonate sediment samples have been studied by INAA (Instrumental Neutron Activation Analysis). Three major Ce A* peaks at ~17, ~53, and ~63 Ma, as well as two minor peaks at 64.6 and 65.2 Ma, were found. The correspondence between observed Ce A* major peaks and the enhanced hydrothermal activity associated with tectonic seafloor rearrangements at these times suggests that the pH of the deep (>600 m) Pacific water was lowered by CO 2 generated by enhanced hydrothermal activity. Manganese and Co, which are highly enriched in hydrothermal solutions, closely follow the Ce A* pattern. This is strong evidence that the Ce A* peaks are related to the elevated levels of hydrothermal activity. The absence of Ce A* changes at the K/T (Cretaceous/Tertiary)-Ir boundary (≡65.0 Ma) suggests that the proposed release of SO 2 and CO 2 by cratering has not significantly lowered the pH of deep ocean water. Our analysis supports only ≲5× increase of atmospheric CO 2 by the putative K/T bolide impact into a ~3 km thick carbonate terrane. We did not observe the pH changes of deep ocean water which could be unambiguously attributed to the volcanic release of SO 2 and CO 2 from the Deccan Trap flows. The ~5 ka (FWHM, Full Width Half Maximum) Ce A* peak ~0.2 m.y. before the K/T-Ir boundary is too short to account for the Deccan Trap eruptions. Of course, our results do not rule out surface water pH changes due to either the Urey-comet or Alvarezasteroid impact or volcanism. The elevated P CO 2 of ~1.9× P CO 2 0 ( P CO 2 0 ≡ present pressure) which started ~0.75 Ma after the K/T-Ir event and lasted for ~2.3 m.y., may be one of the factors responsible for the extended period of extinctions across the K/T boundary for some species. If any species of dinosaurs lived beyond the K/T-Ir event, we predict that they would not have survived the greenhouse effect that very likely occurred between 64.25−62.0 Ma. Our P CO 2 absolute values are in general much lower than the theoretical values suggested by Berner (1990, 1993) during the Phanerozoic except for the Carboniferous and very late Cenozoic where our estimates of ~1.0× P CO 2 0 agree with Berner's. Also, our P CO 2 values are lower than CO 2 estimates derived from paleosol carbonate studies (e.g., Cerling, 1992; Mora et al., 1991).

Intrinsic limitations of standard criteria and methods for delineation of groundwater-source protection areas (spring-head and wellhead protection areas) in carbonate terranes: critical review, technically-sound resolution of limitations, and case study in a Kentucky karst : J. F. Quinlan, J. A. Ray & G. M. Schindel, in: Karst geohazards: engineering and environmental problems in karst terrane. Proc. 5th conference, Gatlinburg 1995,

Intrinsic limitations of standard criteria and methods for delineation of groundwater-source protection areas (spring-head and wellhead protection areas) in carbonate terranes: critical review, technically-sound resolution of limitations, and case study in a Kentucky karst : J. F. Quinlan, J. A. Ray & G. M. Schindel, in: Karst geohazards: engineering and environmental problems in karst terrane. Proc. 5th conference, Gatlinburg 1995,

95/05339 The interaction of acid mine drainage with a carbonate terrane: Evidence from the Obery River, north-central Tennessee

95/05339 The interaction of acid mine drainage with a carbonate terrane: Evidence from the Obery River, north-central Tennessee

The interaction of acid mine drainage with a carbonate terrane: evidence from the Obey River, north-central Tennessee: J. A. Webb & I. D. Sasowsky, Journal of Hydrology, 161(1–4), 1994, pp 327–346

The interaction of acid mine drainage with a carbonate terrane: evidence from the Obey River, north-central Tennessee: J. A. Webb & I. D. Sasowsky, Journal of Hydrology, 161(1–4), 1994, pp 327–346

The interaction of acid mine drainage with a carbonate terrane: evidence from the Obey River, north-central Tennessee

The production of acidic drainage from active or abandoned coal mines is controlled by a complex series of reactions triggered by the oxidation of sulphide minerals. Limestone is frequently used as a neutralizing agent; neutralization occurs both through dissolution of the limestone and exsolution of carbon dioxide from the water. Demonstration of these processes in a natural system is provided by the East Fork of the Obey River, where acidic mine waters travel through conduits in limestone bedrock for several kilometers. Application of discharge-concentration relationships allows tracking of the evolution of these waters within the karst aquifer. Several trends are evident. Virtually all of the iron in the acid mine drainage is precipitated along the flow path within the limestone; the H + released during this reaction, together with the H + generated by pyrite oxidation, is largely neutralized by both exsolution of CO 2 and dissolution of calcite/dolomite in the carbonate bedrock (releasing calcium and magnesium). Nevertheless, the pH is still sufficiently low that all bicarbonate is converted to carbonic acid. Leaching of clay minerals by the acid waters releases small amounts of Al and Si; the species Cl −, F − and NO − 3 are more or less inert. The sulphate concentration substantially increases during evolution of the acid drainage; this seems to be related to an input of sulphate-enriched waters that have dissolved gypsum/anhydrite as they follow deep flow paths within the limestone. Calculations of the relative proportions of H + removed by reaction with calcite/dolomite and CO 2 degassing show that the latter is important in acidic waters that have reacted significantly with limestone and can readily lose CO 2 to the atmosphere. These results indicate that acid neutralization through carbonic acid dissociation, in addition to neutralization through dissolution of carbonates, is a useful method for the commercial a batement of acid mine drainage.

A Survey of Charge‐Balance Errors on Published Analyses of Potable Ground and Surface Waters

AbstractAlthough the charge balance for a single chemical analysis of a water's electrolytes is not a reliable gauge for the accuracy of that analysis, the percent charge‐balance error (%CBE) becomes more credible as a means for evaluating analytical technique when applied to groups of analyses.Just how good are charge balances for chemical analyses of potable ground and stream waters in mainline geological journals? Starting with each journal's first volume, a search found 68 articles in six journals (Appl. Geochem., Chem. Geol, Geochim. Cos. Acta., Ground Water, J. Hydrol, and Water Resources Res.) that had what was deemed to be “complete” analyses of individual samples' major ions. Analyses of brines and hydrothermal solutions were excluded as were “composite” averages. A total of 1,062 %CBEs were computed and tabulated, and the average was 3.99%$ 6.56 (1σ).The average %CBE (by journals) ranges from 1.55% to 9.34%. Quality of %CBE is slightly better for articles published after 1970. Not surprisingly, low ionic strength samples are prone to charge‐balance errors > 10%. Of the 1,062 charge balances calculated, 612 were positive and 450 were negative. Eight of the 68 articles had analyses that were evenly balanced between positive and negative CB errors. There are 21 articles in which the majority of analyses had negative charge‐balance errors. Thus a clear majority (39) of articles had analyses in which there were more positive than negative charge‐balance errors.Systematic error is likely for situations where the analyses in an article are predominantly positive or predominantly negative. The likely reason for the former occurrence is lab determination of alkalinity from the nonacidified field aliquot. Failure to filter samples may cause the latter situation—especially in carbonate terranes. Systematic laboratory errors involving salt standards and dilutions can also yield charge‐balance errors for a suite of analyses that are skewed to either positive or negative errors. The care with which the analyst did his/her job may also impact charge‐balance errors.

Focused fluid flow and Ozark Mississippi Valley-type deposits

Field relations confirm that a tectonic link exists between the Appalachian-Ouachita orogeny and the Mississippi Valley-type Pb-Zn deposits of the Ozarks. Fluids were expelled from both the Reelfoot rift and the Arkoma basin into structurally controlled hydrologic domains. These two source areas fed different fluids into at least six domains and account for regional differences in the ore deposits. Movement of fluids out of the Reelfoot rift also indicates that a foreland basin is not a prerequisite for the migration of fluids. Seismic pumping and distal fluid focusing along faults appears to be the most plausible mechanism to move hot, reactive fluids through a carbonate terrane to a site of ore deposition.

An Investigation of Use of Paleomagnetic Techniques in a Carbonate Terrane--Capitan Reef Complex, Southwestern USA: ABSTRACT

An Investigation of Use of Paleomagnetic Techniques in a Carbonate Terrane--Capitan Reef Complex, Southwestern USA: ABSTRACT

DRY STREAM REACHES IN CARBONATE TERRANES: SURFACE INDICATORS OF GROUND‐WATER RESERVOIRS1

ABSTRACT: The method of identifying dry stream reaches in carbonate terrane as surface indicators of potential ground‐water reservoirs offers a valuable exploration technique that is more rapid and less expensive than traditional exploration methods involving random test drilling. In areas where dry stream reaches occur, subsurface drainage successfully competes with surface drainage, and sheet‐like dissolution openings have developed parallel to bedding creating the ground‐water reservoir. The occurrence and hydraulic characteristics of such reservoirs are highly variable, as attested to by the wide range of well yields.Union Hollow in south‐central Tennessee is the setting for a case study that illustrates the application of the dry stream reach technique. In this technique, dry stream reach identification is based on two types of readily acquired information: (1) remotely sensed black and white infrared aerial photography; and (2) surface reconnaissance of stream channel characteristics. Test drilling in Union Hollow subsequent to identification of the dry reach proved that a localized ground‐water reservoir was present.

The local effects of groundwater pumpage within a fault-influenced groundwater basin, Ash Meadows, Nye County, Nevada, U.S.A.

Large-scale groundwater pumpage and water-level decline data are used in a preliminary attempt to identify the hydraulic connection between several wells and Devils Hole, a small pond in Nye County, Nevada, U.S.A. Results indicate that despite the discontinuous nature of the local aquifers, many wells have good hydraulic connection with Devils Hole. Hydraulic connection between the wells and Devils Hole exhibits a complex spatial variability typical of carbonate terrane. Zones or directions of minimal hydraulic connection may result from fault-controlled structural discontinuities. Zones or directions of enhanced hydraulic connection point to the presence of large-scale groundwater flow through fractures or conduits. The orientation of waterbearing fractures or conduits inferred from this study is qualitatively consistent with the major orientation of local and regional structural features.

Sedimentology and Tectonics of Devonian Nation River Formation, Alaska, Part of Yet Another Allochthonous Terrane: ABSTRACT

Sandwiched between terra incognito of the Yukon Flats, Alaska, and the disrupted cratonal sequences of Yukon Territory, Canada, is a complex array of Proterozoic and Phanerozoic rock units composing a poorly defined group of tectonostratigraphic terranes. The Nation River formation (NRF) is a conspicuous siliciclastic submarine fan complex interbedded in a Paleozoic sequence characterized by deep-water cherts, siliceous shales, and platform to basin-plain carbonates. The NRF ranges from 500 to 2000 m thick. Where the basal part is exposed, NRF overlies the Devonian McCann Hill Chert, a deep-water radiolarian chert sequence. Above the NRF is either another radiolarian chert sequence, the Mississippian Ford Lake Shale, or Permian shallow-water Tahkandit Limestone or Step Conglomerate. NRF lithologies include fine-grained to pebbly turbidites assembled in both thinning- and fining-upward and thickening- and coarsening-upward cycles typical of middle to outer fan settings. Compositionally the grains are principally chert (green, gray, white, black, and rarely red) with minor amounts of vein quartz and quartz sandstone. Most of the chert seems to be replacement chert from a carbonate terrane, though some pebbles yield an Ordovician radiolarian assemblage. Paleocurrent flow directions based on thousands of bottom features (flutes, prods, and grooves) indicate, in present-day coordinates, flow towardmore » the east. Individual azimuth directions are throughout the two easterly quadrants, by 60% of these data indicate flow between 045 and 150/sup 0/. This spread of data is consistent from outcrop to outcrop, indicating that there are no localized block rotations. Easterly flow has also been determined for the overlying Cretaceous units of the Kandik basin (Biederman Argillite and Kathul Graywacke).« less

Mesozoic geology of the Jackson Mountains, northwestern Nevada

Research Article| March 01, 1984 Mesozoic geology of the Jackson Mountains, northwestern Nevada BRANCH J. RUSSELL BRANCH J. RUSSELL 1Department of Geological Sciences, Northwestern University, Evanston, Illinois 60201 Search for other works by this author on: GSW Google Scholar Author and Article Information BRANCH J. RUSSELL 1Department of Geological Sciences, Northwestern University, Evanston, Illinois 60201 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1984) 95 (3): 313–323. https://doi.org/10.1130/0016-7606(1984)95<313:MGOTJM>2.0.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 BRANCH J. RUSSELL; Mesozoic geology of the Jackson Mountains, northwestern Nevada. GSA Bulletin 1984;; 95 (3): 313–323. doi: https://doi.org/10.1130/0016-7606(1984)95<313:MGOTJM>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 The Jackson Mountains Unit, a tectonostratigraphic unit in the Jackson Mountains, northwestern Nevada, includes the Upper Triassic pelagic, carbonate, and volcanogenic sedimentary rocks of the Boulder Creek beds, the Upper Triassic to Middle Jurassic volcanic and volcanogenic sedimentary rocks of the Happy Creek complex, and the Lower Cretaceous sedimentary rocks of the King Lear Formation. The lower Mesozoic rocks of the Jackson Mountains Unit record a transition from sedimentation on a starved lower slope–basinal margin, locally interrupted by prograding deposition from shoaled Mesozoic and possibly Paleozoic carbonate terranes, to the eruption of a proximal intraoceanic magmatic arc. This magmatic arc, represented by the Happy Creek igneous complex, erupted at the margin of North America that lay in northwestern Nevada during early Mesozoic and possibly later times.Between Middle Jurassic and mid-Cretaceous times, the Jackson Mountains Unit was subjected to multiple phases of deformation. Broad, open folding, probably resulting from shortening along a southwest-northeast axis (modern coordinates), affected the region between Middle Jurassic and Early Cretaceous times.During Early Cretaceous times, southeast-northwest shortening produced cleavage, thrusts, and folds. The second phase of shortening, orthogonal to the first, produced local intermountain basins in which the fluvial sediments of the Lower Cretaceous King Lear Formation were simultaneously deposited, on erosional unconformities above lower Mesozoic rocks of the Jackson Mountains Unit, and deformed. Also during the second phase of deformation, late Paleozoic sedimentary rocks were thrust over the Jackson Mountains Unit.A later phase of thrusting affected second phase structures between Early and mid-Cretaceous times. The absence of significant metamorphism and the development of only modest foliation, except near thrust faults, suggest that thrusting in the Jackson Mountains was thin skinned. 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.

Geophysical signatures of fracture traces in a karst aquifer (Florida, U.S.A.)

Abstract Numerous studies have shown that photolinears are associated with zones of increased fracture density, porosity and permeability. In carbonate terranes increased solution along the fractures leads to pronounced surface expressions of subsurface fracture zones, such as sags, sinks and linear depressions. Such features often have several meters of relief and may be as much as 200–300 m wide. R.R. Parizek and others have shown that well yields up to 100 times that of adjacent strata are associated with narrow zones within the surface expressions of the photolinears. These zones are seldom more than a few tens of meters wide, and often only a few meters wide. A study conducted at Cross Bar Ranch wellfield, Pasco County, Florida, demonstrates that several surface geophysical techniques can be used to delineate zones of higher yield within the wider surface expression of photolinears. Seismic refraction and d.c. electrical resistivity proved to be the most useful techniques. The geophysical signatures result from the effect of increased karstic solution on overburden thickness and stratigraphy over the inferred fracture zone. Microgravimetry also appears to be an effective technique, showing a distinctive and linearly consistent gravity low of 0.1–0.3 mGal over the inferred fracture zone. The geophysical signatures of these methods appear to delineate zones of increased fracture density even where overburden thicknesses exceed 20 m. However, due to the density of data needed to delineate these zones, these methods may be practical only in areas where the locations of such zones are critical, and not as a reconnaissance tool.

A lithologic-tectonic framework for the metallogenic provinces of California

The lithologic-tectonic framework of California developed principally during Mesozoic time when various terranes of oceanic crust and island-arc crust were accreted to older sialic crust, resulting in westward growth of the continent. Emplacement of great batholithic masses of granitoid rocks cutting all these crustal types also took place during the Mesozoic period. The discrete tectonostratigraphic terranes that resulted from these events and subsequent Tertiary and Quaternary volcanic events are characterized by specific types of metallic mineral deposits or, in some terranes, by the virtual absence of deposits.Lead-silver-zinc replacement-type deposits are common in the Paleozoic carbonate terrane in the eastern part of the state and occur sporadically elsewhere in the miogeoclinal and cratonal terranes but are absent from the oceanic and island-arc terranes. The vast majority of contact metasomatic tungsten deposits, including all the large ones, are in pendants of miogeoclinal rocks in the Sierra Nevada batholith, but the important Atolia deposits reside in granitoid rocks that invade oceanic terrane. Molybdenum distribution closely follows that of tungsten. All the large contact metasomatic iron deposits in California are in craton and miogeoclinal terranes, but sparse small deposits of this type also occur in island-arc terranes of the northern Sierra Nevada and eastern Klamath Mountains.Lode gold deposits, although widely scattered, show a marked preference for oceanic and island-arc terranes that have been invaded by granitoid plutons. All the major deposits, including late Tertiary bonanza deposits such as Bodie, are in such terranes. It appears that magmatic processes were responsible for mobilizing and transporting the gold, but the metal was perhaps derived from the eugeosynclinal rocks, notably the mafic volcanics.Most mercury deposits are found in the Coast Ranges, where they commonly occur in silica-carbonate rock, an alteration product of serpentinite. The deposits appear to be spatially related to the Coast Range thrust, and the source of the mercury may have been sedimentary rocks of the underlying Franciscan assemblage.Epigenetic mineralization occurred at several different times during the Mesozoic, and again during Miocene and Pliocene time. The timing of mineralization events and the distribution of various deposit types indicate that no broad-scale zoning of epigenetic deposits exists around the Sierra Nevada batholith.Syngenetic deposits are represented mainly by massive sulfides, chert-associated manganese, and chromite. The massive sulfide deposits, with one exception, are restricted to island-arc terranes, and nearly all of these deposits are in silicic volcanic rocks. They are interpreted to be syngenetic with the enclosing rocks, although some redistribution of metals may have occurred after the original deposition. The deposits occur in volcanic sequences of at least five different ages ranging from Early Devonian to Late Jurassic or Early Cretaceous and, along with their enclosing rocks, were probably formed at some distance from their present sites.Chert-associated manganese deposits occur mainly in exotic blocks of oceanic crust in melange and probably formed in fairly deep ocean environments. Chromite is confined to ultramafic rock, much of which occupies suture zones separating various accreted terranes.

Bioclastic Carbonate Facies of Great Barrier Reef, Australia: ABSTRACT

The variations which can be observed in the component compositions of the reefal sediments result from differences in the percentage contributions made by five dominant skeletal types, namely: coral, coralline algae, Halimeda, foraminiferids, and mollusks. The distribution within the reef top environment is controlled partly by the nature of the reefal communities and partly by the production of specific skeletal size modes which are preferentially transported to different depositional environments under a variety of high-energy and/or low-energy hydraulic regimes. The changing nature of the skeletal component composition of the reefal sediments associated with reefs at varying stages of their morphologic evolution is clearly discernible. This allows relations which exist between reef morphology, depositional environments, sediment types, sedimentary facies, and depositional processes to be qualitatively and quantitatively assessed, thereby, providing insight into the behavior of reefal sediments in both a temporal and a spatial context. The commonly occurring sedimentary facies are illustrated and described. They provide an extremely useful basis for paleoenvironmental reconstructions of pre-Holocene reefs or ancient analogs, especially considering the Barrier is one of the few modern carbonate terranes which has exact counterparts in the rock record (e.g., the Devonian Great Barrier Reef of the Canning basin; the Permian Texas reef; the Jurassic-Cretaceous buried reef off the Atlantic Coast; Devonian reef complexes of Canada). End_of_Article - Last_Page 926------------

Evaporitive Limestone: Its Generation and Diagenesis: ABSTRACT

The Calcare-di-base is the only significant carbonate facies of the upper Miocene evaporites of Sicily. It was deposited within an exceedingly saline but marine water body adjacent to older, exposed carbonate terranes which surround the depositional basin. This somewhat dolomitic limestone is commonly cavernous or brecciated at outcrop and contains numerous zones of evident halite dissolution. It also interfingers with thin gypsum stringers in many areas and overlies diatomites of variable thickness. Based on petrographic and isotopic studies, it appears that this limestone was produced by diagenetic processes from an origininal aragonitic mud containing displacive halite hoppers and massive halite zones. Regional variation in carbon-13 (^dgrC13 from 0 to 49 pp PDB) and oxygen-18 (^dgrO18 from +6 to -5 ppt PDB) can be tied to variations in the mineral content of the original sediment and to later diagenetic waters and their organic content. The very negative carbon-13 values and the presence of native sulfur are indicative of the calcitization of gypsum, the by-product of the life processes of sulfate-reducing bacteria. These bacteria utilize part of the available and commonly copious organic matter associated with both the carbonate and with the diatomite. Inversion of the original aragonite to calcite has resulted in expulsion of strontium from the carbonate crystal lattice and the formation of celestite (SrSO4) which now fills the voids left by the dissolution of halite, and the associate pore spaces. End_of_Article - Last_Page 988------------