Beck Spring Dolomite Research Articles

Tonian Low‐Latitude Marine Ecosystems Were Cold Before Snowball Earth

AbstractPrecambrian marine carbonate strata are commonly assumed to have formed in warm‐water carbonate factories due to the temperature dependence of non‐skeletal carbonate precipitation rates. However, some climate models and geological observations suggest that global climate was cool for tens of millions of years prior to the onset of Snowball Earth glaciation at ∼717 Ma, in conflict with common interpretations of pre‐glacial carbonates as warm‐water carbonate factories. We report the occurrence of guttulatic microfabric—a petrographic fingerprint of ikaite, a carbonate mineral that only forms in cold sedimentary environments—in the Beck Spring Dolomite, a carbonate succession deposited in a low‐latitude shallow marine environment between ∼780 and 730 Ma. This interpretation of pre‐glacial carbonate factories aligns cold conditions with vase‐shaped microfossils, possible algal fossils, and molecular clock dates for crown‐group metazoans. Our observations indicate that these marine ecosystems were able to thrive in cold low‐latitude environments millions of years before the Snowball glaciations.

Geologic map of central Panamint Range, California, USA

This detailed geologic map and supplemental digital data set1 examine and demonstrate the complex deformational history and reactivation relationships of the Panamint Range (California, USA), from active transtension of the Walker Lane belt, Miocene extension of the Basin and Range, to multiple Mesozoic events related to subduction, and Neoproterozoic extension. This collection of map data focuses on the geometry, kinematics, and relative timing of deformation to understand the deformation history and effects of structural reactivation. A minor portion of this geologic mapping data was presented in the analysis and figures of Andrew and Walker (2009). The Neogene extension and subsequent dextral transtension deformation has created a complex network of faults via partial reactivation of Mesozoic and Neoproterozoic structures. Structural data show oblique normal slip overprinting earlier normal slip along the western range flank fault of the western Panamint Range. Jurassic and Cretaceous deformation is localized along the western range on the Goldbug fault. The hanging wall of this fault preserves migmatitic fabrics and intense deformation due to Jurassic contraction. The Goldbug fault places Paleoproterozoic to Mesoproterozoic rocks over Neoproterozoic rocks. The Jurassic contraction has top-to-the-northeast relative transport and the more discrete Cretaceous thrust faulting has top-to-the-east transport. A set of Late Cretaceous plutonic rocks and mylonitic gneisses derived from them, occur along the Goldbug fault and demonstrate the reactivated nature of this fault in the Late Cretaceous. New data for the Butte Valley fault show that this fault cuts Late Jurassic plutonic rocks and has normal slip. The Butte Valley fault ends northward at the linked sinistral slip Warm Spring Canyon fault, which was previously interpreted to be an intrusive contact. A previously unrecognized rim syncline structure occurs along the boundary of the Late Jurassic Manly Peak quartz monzonite. Neoproterozoic deformation is difficult to discern due to the overprinting deformations. Numerous Neoproterozoic deformation-related mass wasting deposits can be seen within this formation, including a set of conspicuous allochthonous deposits and clasts of older Beck Spring Dolomite that appear to be frozen in the process of breaking away from intact, normal thickness beds in the Surprise–Happy Canyons divide. This detailed geologic mapping and collection of structural data for the rocks in the central Panamint Range were created using digital in-the-field geographic information systems software running on a field-hardened laptop computer combined with an earlier set of field data that were digitized into the digital georeferenced database. This map is a simplification of detailed geologic mapping data collected at 1:2000–1:6000 scales and reduced to 1:20000 scale. Structural data include kinematic and relative timing of deformation information.

The Tonian Beck Spring Dolomite: Marine dolomitization in a shallow, anoxic sea

The reason for the abundance of dolomite lithologies in Earth's early geological record compared to modern environments remains contentious. This study provides new insight into this Precambrian “dolomite problem” by revisiting one of the most controversial dolomite localities, the Beck Spring Dolomite, of Death Valley, USA. Consistent with some previous studies, petrographic evidence indicates that although the Beck Spring Dolomite now consists almost entirely of dolomite, it was originally precipitated largely as aragonite and high-Mg calcite. Depositional constituents (microbialites and ooids) were likely originally aragonitic, and early marine length-fast cements (now dolomite) are suggested to have precipitated as high-Mg calcite then replaced syntaxially by dolomite. Based on petrographic and geochemical evidence, we suggest that marine dolomitization was the dominant synsedimentary diagenetic process in the unit, and for the most part, involved syntaxial and mimetic replacement. Further, a length-slow fibrous dolomite generation was precipitated during the later stages of marine diagenesis as a primary marine dolomite cement. This is indicated by the length-slow crystallographic structure of the cement and from its preserved geochemical and cathodoluminescence growth zonation. This new evidence for Tonian marine dolomite precipitation reinforces the idea of Precambrian marine environmental conditions, including the chemical composition of seawater, promoting dolomite formation at this time. The trace metal geochemical composition of well-preserved marine components, especially dolomite marine cements, reveals information about redox conditions in this Tonian shallow seawater. In terms of rare earth element geochemistry, the Beck Spring Dolomite has no significant Ce anomaly, and a ubiquitous positive Eu anomaly, consistent with widespread oceanic anoxia during deposition. Furthermore, the relatively low levels of iron and chalcophile elements Co, Cu, Pb and Zn in marine components compared to other Neoproterozoic carbonates suggest euxinic conditions (both anoxic and sulphidic) prevailed during deposition of the Beck Spring Dolomite.

Tectonostratigraphic evolution of the c. 780–730 Ma Beck Spring Dolomite: Basin Formation in the core of Rodinia

Abstract The Beck Spring Dolomite is a mixed carbonate–siliciclastic succession exposed in Death Valley, California, that was deposited between 780 and 717 Ma. Along with its bounding units, the Horse Thief Springs Formation below and unit KP1 of the Kingston Peak Formation above, the Beck Spring Dolomite were deposited in one of the ChUMP (Chuar–Uinta Mountains–Pahrump) basins with subsidence commonly attributed to the nascent rifting of Rodinia. These pre-Sturtian successions preserve eukaryotic microfossil assemblages, diverse microbialites, and large carbon isotope anomalies directly below Sturtian-age glacial deposits. Here we present new geological mapping, measured stratigraphic sections, carbon isotope chemostratigraphy and detrital zircon geochronology from the Beck Spring Dolomite and its bounding units. The carbon isotope excursion at the top of the Beck Spring Dolomite has previously been attributed to meteoric diagenesis associated with karst breccias, but here we demonstrate that these breccias are instead mass flow deposits that formed during deposition of the Kingston Peak Formation and that the carbon isotope excursion is not only reproducible throughout the basin, but is associated with transgression rather than regression and exposure. In addition, we refine local correlations and discuss the use of chemostratigraphic curves from these units for regional and global correlations. The Beck Spring Dolomite was deposited during the second of three distinct basin-forming events recorded in the Pahrump Group with basin inversion occurring between each event. The presence of syn-sedimentary faults, the character of the lateral facies change and detrital zircon provenance analyses indicate that the Beck Spring Dolomite fringed a coeval palaeo-high to the south in a tectonically active basin. Detrital zircon age distributions in the Beck Spring Dolomite show sharp probability peaks at c. 1200, 1400 and 1800 Ma, consistent with local sources to the SW in the Mojave block rather than transcontinental rivers. The c. 1800 Ma probability peak is less prominent in the KP1 samples. In addition, KP1 also records slump folding and is overlain by an unconformity. We suggest that these features are consistent with the emergence of a local fault to the NE. Deposition of the Beck Spring Dolomite and bounding units do not record evidence of incipient rifting of the western margin of Laurentia but instead reflect a distinct and separate tectonothermal event. Supplementary material: Carbon (δ 13 C) and oxygen (δ 18 O) isotopic measurements, detrital zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) data, detrital zircon sample information and data from reference materials used for LA-ICPMS analyses are available at http://www.geolsoc.org.uk/SUP18823 .

Origins of Microbial Microstructures In the Neoproterozoic Beck Spring Dolomite: Variations In Microbial Community and Timing of Lithification

Abstract Microbialite microstructures form at the scale of microbial processes and can preserve evidence of growth, degradation, and lithification in microbial communities. Understanding how these processes are preserved in microstructures will provide insights into microbial ecosystems and how microbial communities have evolved through time. Microbialites in the Neoproterozoic Beck Spring Dolomite, southern California, U.S.A., contain microstructures that record variations in the morphology of microbial communities as well as the timing of degradation versus lithification in the communities. These processes are represented by five end-member microbial microstructures: 1) distinct laminated microstructure; 2) diffuse laminated microstructure; 3) distinct clotted microstructure; 4) diffuse and regular clotted microstructure; and 5) diffuse and irregular clotted microstructure. These microstructures are distinguished by fabrics defined by organic inclusions. In distinct microstructures, organic inclusions sharply define microbial features, such as laminae that are laterally continuous for millimeters or 50–200 micron diameter microdots. The contacts between distinct microstructures are always sharp. In diffuse microstructures, zones of organic inclusions lack distinct boundaries; neighboring laminae and clots grade into each other within tens of microns, and boundaries between microbial structures and encasing cements are indistinct. Diffuse microstructures grade into each other and into distinct microstructures in a systematic way that suggests that differences are due to variations in the timing of mineralization relative to growth and degradation of microbial communities. Sharply defined textures in distinct microstructures suggest that lithification started during growth of the microbial community, and the close spatial association but lack of intergrading between distinct laminated and distinct clotted microstructures suggests that they formed from two distinct microbial communities that lived in the same environment. By contrast, diffuse microstructures formed where lithification occurred after the onset of decay and degradation of the primary growth structure. The continuum from distinct microbial textures through more diffuse and irregular textures documents variations in the timing of lithification relative to growth and decay of the microbial community. Thus, where petrographic preservation is extremely good, such as in the Beck Spring Dolomite, sub-millimeter-scale microbial microstructures record a gradation between growth and degraded microbial structures depending on the relative timing of lithification and heterotrophic decay.

INFLUENCE OF GAS PRODUCTION AND FILAMENT ORIENTATION ON STROMATOLITE MICROFABRIC

Alternating light-dark laminae within stromatolites have been attributed to a phototactic response of the constituent microbial communities, whereby filaments orient vertically during the day and recline at night. This study examines the orientation of cyanobacterial filaments within a laminated siliceous stromatolite from a Yellowstone National Park hot spring to identify the controls on microfabric development and whether phototaxis plays a role. Results indicate that filament orientation is predominantly perpendicular or parallel to lamination, even when laminae are steeply inclined. Thus, phototaxis is not a significant control of microfabric development in these stromatolites. Vertical aspects of the fabric are dominated by hourglass-shaped filament bundles (hourglass structures) adjacent to rounded pores, rather than being defined by individual filaments. The rounded pores likely represent oxygen-rich bubbles generated during photosynthesis. Upon stabilization by filaments, upward buoyancy of the bubbles rotated the bundles toward a vertical orientation. Thus, vertical aspects of the fabric in this stromatolite result from buoyancy forces rather than phototaxis. Examples from the Neoproterozoic Beck Spring Dolomite reveal that similar subvertical hourglass structures are present in the rock record and may be better preserved than individual filaments. The presence of rounded pores (fenestrae) and hourglass structures in ancient microbialites, here termed the hourglass-associated fenestral fabric, can serve as an indication of biogenic influence in stromatolites, especially in the absence of preserved filaments, and may be an indication of oxygenic photosynthesis.

Microbialites of the Neoproterozoic Beck Spring Dolomite, Southern California

AbstractAncient microbialites reflect interactions between microbial communities and environmental conditions. However, evaluating the relative roles of microbial community processes and environmental influences on microbialite morphology and internal fabric in the rock record can be challenging. The Neoproterozoic Beck Spring Dolomite preserves diverse microbialites, and thus provides an opportunity to explore the factors that influenced microbialite development locally. Stromatolitic, thrombolitic and composite microbialites are abundant in subtidal to upper intertidal carbonates in the Beck Spring Dolomite. Thrombolitic and composite microbialites have not been recognized previously in this unit, but compose much of the newly defined thrombolitic member. Stratigraphic relationships demonstrate that these three types of microbialites formed in close spatial and temporal association in subtidal to intertidal environments. The relative proportions and distributions of stromatolitic and thrombolitic microbialites vary with depositional environment; stromatolitic microbialites dominate in deeper intertidal to subtidal facies, whereas thrombolitic textures are more abundant in upper intertidal facies. Composite microbialites, composed of intermingled clotted and laminated textures, formed in all environments but are most abundant in intertidal facies. The broad environmental distribution of stromatolitic, thrombolitic and composite microbialites and the intermingling of textures suggest that laminated and clotted textures reflect diverse microbial community morphologies rather than environmental variations. Furthermore, the ca 750 Ma age of thrombolitic microbialites in the Beck Spring Dolomite requires that they formed without the influence of calcimicrobes or metazoans colonizing and grazing the microbial mat surface. Thus, these thrombolites provide further evidence that the biostratigraphic distribution of thrombolites cannot be uniquely attributed to evolution of calcifying and grazing organisms in the earliest Cambrian, and that older microbial communities were capable of producing clotted textures.

The Origin of the Millimeter-Scale Lamination in the Neoproterozoic Lower Beck Spring Dolomite: Implications for Widespread, Fine-Scale, Layer-Parallel Diagenesis in Precambrian Carbonates

Abstract The Neoproterozoic lower Beck Spring Dolomite exhibits a prominent, millimeter-scale, light–dark, planar lamination commonly interpreted as a primary depositional fabric. Microscopic examination reveals that the dark laminae are composed of micrite and contain a micrometer-scale, wavy fabric, whereas the light laminae are composed of relatively coarse-crystalline pseudospar that engulfs, surrounds, and crosscuts the dark laminae. Nearly all of the light laminae surround, or occur in proximity to, elongate (long axis fabric-parallel), spar-filled cavity structures. The average δ18O values for dark and light laminae are −1.8‰ (std. dev. 1.6) and −3.0‰ (std. dev. 1.3) VPDB, respectively. Cavity-filling cements are even more depleted in 18O, with an average isotopic value of −6.2‰ (std. dev. 1.1). These characteristics suggest that the coarse-crystalline, light laminae are the result of the preferential, aggrading neomorphism of a thinly laminated, micritic host rock and that the (now cemented) cavity structures acted as conduits for the fluid responsible for recrystallization. The oxygen isotope distributions are consistent with progressive recrystallization by an 18O-depleted fluid, likely of meteoric origin. Therefore, we interpret the prominent millimeter-scale, light–dark layering in the lower Beck Spring Dolomite to result from widespread, laminae-parallel, diagenetic processes—perhaps controlled by fine-scale layer-parallel differences in initial porosity and permeability—and not from primary sedimentary processes. Extensive, layer-parallel diagenesis on such a fine scale is rare in Phanerozoic shallow marine deposits, due to disruption by bioturbation. However, before the advent of metazoan burrowing, fine-scale, layer-parallel fluid flow would have been more common. Thus, the diagenetic fabric of the lower Beck Spring Dolomite may represent a mode of carbonate diagenesis restricted to the Precambrian. Similar diagenesis may also occur in Phanerozoic environments devoid of vertical bioturbation.

Sedimentology, diagenesis, and stratigraphic occurrence of giant ooids in the Ediacaran Rainstorm Member, Johnnie Formation, Death Valley region, California

The Rainstorm Member of the late Neoproterozoic Johnnie Formation contains several distinctive features of possibly regional to global significance—the Johnnie Oolite, an incised valley with a carbonate breccia fill, and an enigmatic bed of giant ooids (the Rainstorm giant ooids). Although the Johnnie Oolite and the incised valley occur in most Johnnie Formation outcrops scattered throughout the Death Valley region, the giant ooids have only been observed in one area, restricted to the southern Nopah Range. If these giant ooids were deposited as part of the Rainstorm Member, they would represent the youngest known occurrence of Neoproterozoic giant ooids and the only such unit deposited after the last Precambrian glaciation (the Gaskiers). However, since the giant ooid bed is associated with the incised valley fill, it could simply be a large block of oolite derived from the stratigraphically lower Beck Spring Dolomite, known to contain giant ooids. This study uses mapping, petrography, and isotope geochemistry to compare giant ooids from the Rainstorm Member and the Beck Spring Dolomite, with the goal of determining the provenance of the Rainstorm giant ooids. All lines of evidence suggest that the Rainstorm giant ooids form a unit distinctly different from those of the Beck Spring Dolomite. Carbon isotope values from the ooid bed indicate that it was probably deposited after the Shuram δ 13C excursion, making this the youngest known Precambrian giant ooid unit. The most plausible explanation for this bed is that it is a raft derived from a so far unobserved pre-incision Rainstorm Member giant ooid bed, which would also have been the source of many of the giant ooid clasts in the incision breccia.

Johnnie

The Rainstorm Member of the late Neoproterozoic Johnnie Formation contains several distinctive features of possibly regional to global significance—the Johnnie Oolite, an incised valley with a carbonate breccia fill, and an enigmatic bed of giant ooids (the Rainstorm giant ooids). Although the Johnnie Oolite and the incised valley occur in most Johnnie Formation outcrops scattered throughout the Death Valley region, the giant ooids have only been observed in one area, restricted to the southern Nopah Range. If these giant ooids were deposited as part of the Rainstorm Member, they would represent the youngest known occurrence of Neoproterozoic giant ooids and the only such unit deposited after the last Precambrian glaciation (the Gaskiers). However, since the giant ooid bed is associated with the incised valley fill, it could simply be a large block of oolite derived from the stratigraphically lower Beck Spring Dolomite, known to contain giant ooids. This study uses mapping, petrography, and isotope geochemistry to compare giant ooids from the Rainstorm Member and the Beck Spring Dolomite, with the goal of determining the provenance of the Rainstorm giant ooids. All lines of evidence suggest that the Rainstorm giant ooids form a unit distinctly different from those of the Beck Spring Dolomite. Carbon isotope values from the ooid bed indicate that it was probably deposited after the Shuram δ13C excursion, making this the youngest known Precambrian giant ooid unit. The most plausible explanation for this bed is that it is a raft derived from a so far unobserved pre-incision Rainstorm Member giant ooid bed, which would also have been the source of many of the giant ooid clasts in the incision breccia.

Stratigraphic investigations of carbon isotope anomalies and Neoproterozoic ice ages in Death Valley, California

An unusual richness of biogeochemical events is recorded in Neoproterozoic‐ Cambrian strata of the Death Valley region, California, United States. Eight negative carbon isotope (d13C) excursions are found in carbonate units between 1.08 Ga and the Precambrian/Cambrian boundary; four of these excursions occur in carbonates that contain textural features similar to those found globally in postglacial ‘‘cap carbonates’’ (including one or more of the following: laminite with rollup structures, apparent ‘‘tube rocks,’’ seafloor precipitates, and sheet-crack cements). However, only two of these units, the Sourdough limestone member of the Kingston Peak Formation and the Noonday Dolomite, rest directly upon glacial strata. The basal Beck Spring Dolomite and the Rainstorm Member of the Johnnie Formation each contain negative excursions and cap-carbonate‐like lithofacies, but do not rest on known glacial deposits. If the negative d13C excursions are assumed to record depositional processes, two equally interesting hypotheses are possible: (1) The Death Valley succession records four glacial pulses in Neoproterozoic time, but glacial units are not preserved at two stratigraphic levels. (2) Alternatively, other global oceanographic processes can cause negative excursions and cap-carbonate‐ like facies in addition to, or independent of, glaciation.

Stable isotope variations in the Neoproterozoic Beck Spring Dolomite and Mesoproterozoic Mescal Limestone paleokarst: Implications for life on land in the Precambrian

Proterozoic karst events lowered δ13C values by as much as 11‰ for the 800 Ma Beck Spring Dolomite, California, and as much as 8.5‰ for the 1.1 Ga Mescal Limestone, central Arizona, relative to the originally deposited carbonate. The 13C changes are attributed to input of 13C-depleted organic CO2 derived from photosynthetic organisms that colonized the ancient land surface. The large isotopic shift and its presence at two separate localities suggest that Proterozoic karst surfaces were colonized by significant photosynthetic communities with phytomasses possibly approaching those of today.

Life on Land in the Precambrian

Microfossils have been discovered in cavity-fill and replacement silica that occurs between chert-breccia clasts in 1200-million-year-old paleokarst at the top of the Mescal Limestone, central Arizona, and in approximately 800-million-year-old paleokarst at the top of the Beck Spring Dolomite, southeastern California. Microbial communities on Precambrian (>550 million years ago) land may have been extensive enough to affect weathering, erosion, sedimentation, and geochemical processes.

Experimental and Natural Mimetic Dolomitization of Aragonite Ooids

ABSTRACT To better understand fabrics in dolomitized carbonates, aragonitic ooids were replaced with dolomite in laboratory experiments. Recent ooids from Cat Cay were reacted in a solution of 0.5 M MgCl2 and 0.4 M CaCl2 in Teflon-lined stainless steel bombs at 200°C and equilibrium vapor pressure. X-ray diffraction and petrographic study were used to characterize compositional and textural changes during dolomitization. Reaction progressed from aragonite through unordered Ca-Mg carbonate to ordered dolomite within 180 hr. Reaction products are texturally identical irrespective of composition and order. We believe that near-stoichiometric compositions and ordering were achieved by microscale dissolution and reaction of intermediate phases. Dolomitization occurred by dissolution of aragonite concurrent with precipitation of rhombohedral Ca-Mg carbonate. Dolomitization occurs first at ooid surfaces, where microsucrosic crystals precipitated and coalesced to form a rind. With time, reaction preferentially proceeded into discrete zones of ooid microporosity, where dolomite successively replaced algal borings, outer ooid cortices, and inner ooid cortices. Precipitation and coalescence of dolomite crystals in cortical laminae closely mimics original ooid structure. Comparison of experimental fabrics with calcitized, partially dolomitized, and completely dolomitized aragonite ooids of the late Proterozoic Beck Spring Dolomite indicates that mimetic concentric fabric originates by aragonite dissolution and early dolomite precipitation within porous cortical laminae. Importantly, these experimental and natural replacement fabrics show that fine-scale dolomite ooid fabrics are not indicative of primary dolomite precipitation. Isotopic compositions of Beck Spring mimetic dolomite ooids are enriched relative to other dolomitized metastable marine components, including acicular cement (aragonite), carbonate mud (aragonite-rich), and columnar cement (magnesian calcite). These data suggest that (1) initial reaction of aragonite ooid cortices occurred early in the diagenetic history in marine-dominated fluid, and (2) selective reaction of aragonite ooid cortices was promoted by a combination of high solubility, high cortical porosity, and high reactant surface area relative to other metastable components. Accordingly, textural and chemical data gathered from mimetic concentric dolomite ooids may be useful indicators of precursor aragonite and early dolomitization events.

Petrology, petrochemistry, and stromatolites of the Middle to Late Proterozoic Beck Spring Dolomite, eastern Mojave Desert, California

The Beck Spring Dolomite is the medial unit of the Middle to Late Proterozoic Pahrump Group, the oldest sequence of sedimentary rocks in eastern California. Stratigraphic sections of the Beck Spring Dolomite examined in the eastern Mojave Desert and Death Valley regions consist of four members. These are, in ascending order, a lower cherty member, a lower laminated member, an oolitic–pisolitic member, and an upper cherty member. More than 80% of the Beck Spring Dolomite is algal-laminated dolomite with a possible Middle to Late Riphean stromatolite assemblage characterized by cf. Conophyton, eroded, irregular columnar forms similar to Kussiella or Baicalia, and several types of stratiform Stratifera. Petrographic, X-ray diffraction, and atomic absorption spectroscopic analyses indicate that the formation is composed of well-ordered replacement dolomite with less than 25% acid-insoluble residue. Concentrations of Fe and Mn are two to six times higher in the algal-laminated members than in the oolitic–pisolitic member, whereas the concentrations of Ca, Mg, Ba, Sr, Na, and K show no systematic variations. Stratigraphic relationships, primary and secondary sedimentary structures, petrology, and stromatolite assemblages suggest deposition during Middle to Late Proterozoic time on a platform that most likely included offshore shoals, restricted lagoons, and broad tidal flats with ponds, channels, and levees.

Meteoric Stabilization and Preservation of Limestone Within Late Proterozoic Beck Spring Dolomite of Eastern California: ABSTRACT

Petrographic and geochemical study of limestone, silicified carbonate, and dolostone indicates that meteoric diagenetic processes and paleogeographic position played important roles in the stabilization and preservation of limestone within the pervasively dolomitized Beck Spring Dolomite. The authors conclude that Beck Spring marine cement allochems precipitated as aragonite and high-magnesium calcite. In the vicinity of Saratoga Spring, these metastable mineralogies were dissolved, eroded, stabilized to calcite, and replaced by silica through interaction with meteoric waters. Early stabilization of limestone components along the margin of the Amargosa basin apparently decreased susceptibility to subsequent dolomitization.

Diagenesis of Late Proterozoic Carbonates: The Beck Spring Dolomite of Eastern California

ABSTRACT Petrographic and compositional attributes of limestone and dolostone in the Beck Spring Dolomite of eastern California indicate that late Proterozoic mineralogies, isotopic compositions, and diagenetic processes were similar to those inferred for younger Phanerozoic marine carbonate sequences. Allochems in limestones include strontium-rich ooids with well-preserved concentric banding and cortices of equant neomorphic calcite that commonly exhibit dissolution and compaction features; cements include microdolomite-rich columnar marine calcite and equant spar. Calcites are depleted relative to dolomites and become enriched with stratigraphic depth. Acicular and columnar marine cement and allochems in dolostones display fabrics identical to those in limestones but are isotopically distinct. Dolomite ooids, columnar and acicular cement, crystal silt, and early luminescent spar cement from four stratigraphic sections define overlapping positive oxygen-carbon covariant fields that trend toward depleted calcite compositions. Early luminescent dolomite spar cement is unconformably overlain by depleted ferroan dolospar. Fabric data suggest that marine cement and ooids were precipitated as magnesian calcite and aragonite. Isotopic data suggest that metastable components locally stabilized to calcite during subaerial exposure while laterally, dolomitization took place in mixed marine-meteoric waters. Early dolospar precipitation began during replacive dolomitization, and late ferroan dolospar precipitated during burial. Lack of evidence for hypersaline conditions suggests that late Proterozoic normal marine carbonate isotopic composition was -0.5 ± 2.018O and +5.3 ± 0.5913C.

Late Precambrian tectonism in the Kingston Range, southern California

Research Article| January 01, 1986 Late Precambrian tectonism in the Kingston Range, southern California J. Douglas Walker; J. Douglas Walker 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Search for other works by this author on: GSW Google Scholar David W. Klepacki; David W. Klepacki 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Search for other works by this author on: GSW Google Scholar B. C. Burchfiel B. C. Burchfiel 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Search for other works by this author on: GSW Google Scholar Author and Article Information J. Douglas Walker 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 David W. Klepacki 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 B. C. Burchfiel 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1986) 14 (1): 15–18. https://doi.org/10.1130/0091-7613(1986)14<15:LPTITK>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 J. Douglas Walker, David W. Klepacki, B. C. Burchfiel; Late Precambrian tectonism in the Kingston Range, southern California. Geology 1986;; 14 (1): 15–18. doi: https://doi.org/10.1130/0091-7613(1986)14<15:LPTITK>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 SocietyGeology Search Advanced Search Abstract Speculation on the early history of the Cordilleran geosyncline of North America has relied in part on interpretation of the tectonic setting of the Precambrian Pahrump Group. Recent mapping in the Kingston Range resulted in new data to aid our speculation. Specifically, Precambrian tectonic events involving folding and later low-angle normal faulting affected the Crystal Spring Formation, Beck Spring Dolomite, and lower part of the Kingston Peak Formation. Unconformably overlying these units is a relatively undeformed conglomerate. Diamictite occurs both above and below the unconformity; hence, Cordilleran-wide correlations of the Kingston Peak Formation based on diamictite require reassessment.The nature of boundaries, localized deformation, and outcrop distribution of the Pahrump Group suggest that these rocks may be preserved in transtensional basins associated with strike-slip faulting, possibly occurring during development of the late Precambrian continental margin. 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.

Diagenesis, Geochemistry, and Origin of a Precambrian Dolomite: the Beck Spring Dolomite of Eastern California

The Beck Spring Dolomite is a platform carbonate sequence dominated by cryptalgal laminites with some stromatolites and grainstones and is inferred to have been deposited in shallow subtidal to intertidal environments. Cavity structures, where the cements occur, include birdseyes, laminoid fenestrae, stromatactis, sheet cracks, and neptunian dikes. Four types of synsedimentary-early diagnetic fibrous dolomite are distinguished. Type A forms isopachous fringes consisting of length-slow acicular to columnar pseudopleochroic crystals with fine, substrate-parallel color banding. The dolomite is mostly nonluminescent with some bright blotches, and inclusion trains, define acicular crystallites within the columnar crystals. Type B consists of length-slow columnar crystals with distinctive bladed relic structures, defined by inclusions and nonluminescence, surrounded by inclusion-free and brightly luminescent dolomite. Type C consists of bladed to wedge-shape pseudopleochroic crystals with undulose extinction (mostly fascicular optic); inclusion trains define acicular crystallites within crystals, and forms mammillated crusts and botryoids. Type D fibrous dolomite forms thin layers of acicular crystals, with a distinctive geometry of the cement layers thickening into the corners of cavities. Types B and C contain relic structures indicating replacement, but types A and D have fabrics which could be primary. The depositional components (pisolites and micrite) and some fibrous dolomites have oxygen isotopic values around -2 per mil versus PDB, compatible with a marine origin. However, Na (around 220 ppm) and Sr (around 65 ppm) contents are very low, suggesting precipitation from dilute waters. Meteoric-marine mixing-zone dolomitization of CaCO 3 sediments and early cements. Or primary marine precipitation of poorly ordered calcian dolomite and early diagenetic stabilization to stoichiometric dolomite. The dolomite spar has very negative values of delta 18 O (-9.6 per mil versus PBD) and is more depleted in Na (34 ppm) and Sr (47 ppm), reflecting precipitation from meteoric waters of very low ionic strength during burial diagenesis. A replacement origin for the dolomite of the depositional components and fibrous cements can be reconciled with the lack of obliterative replacement fabrics invoking an original high-Mg calcite mineralogy.--Modified journal abstract.

Precambrian dolomites: Petrographic and isotopic evidence that they differ from Phanerozoic dolomites

Dolomites are more common in Precambrian than in Phanerozoic rocks and, from a petrographic-isotopic study of the middle to upper Proterozoic Beck Spring Dolomite of eastern California, it is suggested that Precambrian dolomites are different from those of the Phanerozoic. Petrographically, all fabric details of Precambrian dolomites studied are preserved, just as if they were limestones, and isotopically, they show the same trend as limestones: increasingly negative δ I8 O and to a lesser extent increasingly light δ I8 C, from depositional components, through early fibrous dolomite cements to later dolomite spar cements. The data suggest that in Precambrian time dolomite was the principal carbonate mineral precipitated from seawater and during diagenesis, and this implies that Precambrian seawater was different from that of the Phanerozoic.