Non-skeletal Carbonate Grains Research Articles

Ichnologic note first occurrence of the microcoprolite Palaxius salataensis in the Aptian (Lower Cretaceous) of northeast Mexico and its paleoecological implications

Microcoprolites of Palaxius salataensis are non-skeletal carbonate grains produced by callianassid shrimps. This ichnospecies is recognized by four longitudinal canals arranged around a bilateral plane of symmetry, with crescent outline in cross-section. Palaxius salataensis has been previously reported from other countries and in strata from the late Paleozoic to the Mesozoic. In this study, we describe the first Aptian record of the ichnospecies, the first Cretaceous occurrence in North America, and the fifth occurrence worldwide. Palaxius salataensis was found in a packstone-grainstone with abundant pellets and common gastropod shell fragments and subangular, poorly sorted intraclasts. The paleoenvironment was characterized by shallow-waters with high energy conditions, occurring between fair-weather wave base and storm wave base, close to the platform margin, where well-oxygenated and nutrient-rich conditions predominated. The microcoprolite-rich bed of the La Huasteca Canyon occurs on top of an oolitic grainstone interval in the upper levels of the Cupido Formation, interpreted as the termination of the early Aptian Oceanic Anoxic Event 1a (OAE 1a). The presence of P. salataensis in the upper levels of the Cupido Formation is directly related to the availability of organic matter rather than to oxygen recovery.

Redox conditions and authigenic mineralization related to cold seeps in central Guaymas Basin, Gulf of California

Highlights • In cold seeps of Guaymas Basin, aragonite, barite and pyrite precipitated from modified seawater. • Aragonite is highly depleted in 13C suggesting formation via anaerobic oxidation of methane. • Barite formed through mixing of reducing, Ba-rich seep fluids with a 34S-rich sulfate pool. • Pyrite framboids formed under anoxic-sulfidic water via microbial sulfate reduction. Abstract Authigenic carbonate crusts, surface muds and bivalve shell fragments have been recovered from inactive and active recently discovered cold seep sites in central Guaymas Basin. In this study, for first time, redox conditions and fluid sources involved in mineral precipitation were investigated by analyzing the mineralogy and textures of surface samples, along with skeletal contents, and C, O and S isotopes variations. The δ13C values of aragonitic bivalve shells and non-skeletal carbonate from some surface muds (1‰ to −3.7‰ V-PDB) suggest that carbonate precipitated from ambient dissolved inorganic carbon, whereas fibrous aragonite cement and non-skeletal carbonate from other sites are highly depleted in 13C (down to −47.6‰ V-PDB), suggesting formation via anaerobic oxidation of methane, characteristic of methane seepage environments. δ18O in most of the carbonates varies from +1.4‰ to +3.2‰ V-PDB, indicating that they formed from slightly modified seawater. Some non-skeletal carbonate grains from surface muds have lower δ18O values (−12.5‰ to −8.2‰ V-PDB) reflecting the influence of 18O-depleted pore water. Size distribution of pyrite framboids (mean value: 3.1 μm) scattered within diatomaceous sinter suggests formation from anoxic-sulfidic bottom waters. δ34S in pyrite is of −0.3‰ V-CDT compared to +46.6‰ V-CDT in barite, thus implying a fluid sulfate−sulfide fractionation of 21.3‰ that argues in favor of microbial sulfate reduction as the processes that mediated pyrite framboid formation, in a semi-closed system. Barite formation occurred through the mixing of reducing and Ba-rich seep fluids with a 34S-enriched sulfate pool that resulted from microbial sulfate reduction in a semi-closed system. The chemical composition of aragonite cement, barite and pyrite suggest mineral precipitation from modified seawater. Taken together, our data suggest that mineralization at the studied seep sites is controlled by the mixing of seawater with minor amounts of hydrothermal fluids, and oxygen-depleted conditions favoring anaerobic microbial processes.

Sedimentary Facies of Bora Bora, Darwin's Type Barrier Reef (Society Islands, South Pacific): The Unexpected Occurrence of Non-Skeletal Grains

Abstract “Barrier reefs, when encircling small islands, have been comparatively little noticed by voyagers; but they well deserve attention. In their structure they are little less marvelous than atolls, and they give a singular and most picturesque character to the scenery of the islands they surround.” (Darwin 1842, p. 2) Sediment sampling revealed six modern sedimentary facies in the barrier-reef system around the volcanic island of Bora Bora including mixed skeletal packstone, peloidal packstone, Halimeda-rich packstone to grainstone, coralgal grainstone, wackestone, and mudstone. Whereas sediments rich in corals and coralline algae are abundant on reefs and carbonate shoals, mud-rich sediments with mollusks and foraminifera occur in deep lagoons (15–40 m). Carbonate mud is composed largely of aragonite needles, probably derived from codiaceans like Halimeda, and coccoliths in moderate abundances. Despite the proximity to a volcanic island, the content of siliciclastics in the sediments is low (< 10% on average), which is a consequence of the absence of permanent water courses. In addition to skeletal grains, non-skeletal grains (cemented fecal pellets; aggregates) occur in high abundance (30–50%) in shallow lagoons (< 5 m) on the eastern side of Bora Bora. This observation opposes the widely-held concept that non-skeletal carbonate grains are abundant on platforms from the Atlantic Ocean but virtually missing in recent Indo-Pacific reefs and carbonate platforms. In Bora Bora, non-skeletal grains and aggregates apparently form in warm, shallow (< 5 m), moderately agitated areas with relatively low sedimentation rates. In addition, early cementation is probably favored by elevated alkalinity and pH in surface waters in this part of the south Pacific. In the same area, some 900 km to the southwest of the study area, ooids and peloids have recently been discovered in the shallow lagoon of Aitutaki, Cook Islands. More occurrences of abundant non-skeletal grains probably exist in the Indo-Pacific region; however, too few locations have been systematically studied. A comparison of the new Bora Bora data with other modern reefs and platforms shows that sediment occurrence and distribution exhibits basic patterns in that marginal areas are dominated by coarse-grained coralgal sediment whereas lagoons and platform interiors are characterized by mollusk, foram, Halimeda, and non-skeletal grains, and fines. Still, grain abundances may vary significantly among locations, and again suggest the need for more studies in order to fully understand the factors controlling occurrence and distribution of modern sediments.

Facies models of a shallow-water carbonate ramp based on distribution of non-skeletal grains (Kimmeridgian, Spain)

The internal facies and sequence architecture of a Late Jurassic (Late Kimmeridgian) shallow carbonate ramp was reconstructed after the analysis and correlation of 17 logs located south of Teruel (northeast Spain). The studied rocks are arranged in five high-frequency sequences A–E (5–26 m thick) bounded by discontinuities traceable across the entire study area (20 × 25 km). Facies analysis across these sequences resulted in the reconstruction of three sedimentary models showing the transition from interior ramp environments (i.e., lagoon, backshoal, and shoal) to the progressively deeper foreshoal and offshore areas. Coral-microbial reefs (meter-sized patch and pinnacle reefs) have a variable development throughout the sequences, mostly in the foreshoal and offshore-proximal environments. The preferential occurrence and down-dip gradation of non-skeletal carbonate grains has been evaluated across the three models: low-energy peloidal-dominated, intermittent high-energy oolitic-dominated and high-energy oolitic–oncolitic dominated. The predominance of these non-skeletal grains in the shoal facies was mainly controlled by the hydrodynamic conditions and spatial heterogeneity of terrigenous input. The models illustrate particular cases of down-dip size-decrease of the resedimented grains (ooids, peloids, oncoids) due to storm-induced density flows. Offshore coarsening of certain particles (intraclasts, oncoids) is locally observed in the mid-ramp areas favorable for microbial activity, involving coral-microbial reef and oncoid development. The observed facies variations can be applicable to carbonate platforms including similar non-skeletal components, where outcrop conditions make the recognition of their three-dimensional distribution difficult.

Permian limestone in the southeastern Bowen Basin, Queensland: an example of temperate carbonate deposition

During the Permian the Bowen Basin, a foreland basin in eastern Australia, was influenced by cold to cool-temperate climatic conditions at a paleolatitude of 60 °S. Limestones are rare in the sequence except in the southeastern Bowin Basin area where two limestone-bearing sequences are present. The limestones are mainly skeletal grainstones and rarer packstones; skeletal grains include crinoids, bryozoans, brachiopods, molluscs, ahermatypic corals, foraminifera and sponge spicules. Crinoid remains are dominant, but brachiopod-rich and coral-rich limestones are present locally. Non-skeletal carbonate grains are absent from the limestones. Terrigenous components range from negligible to dominant. Comparison of the limestones with others in the Permian sequences in eastern Australia reveals a consistency in sedimentary style, the only variations being in the relative proportions of the skeletal fragments. The Permian limestones share similar characteristics with temperate Cenozoic limestones of New Zealand, suggesting that differences in carbonate sedimentation between tropical and non-tropical regions have been consistent through time and reflect real sedimentological differences.

A strontium isotopic study of Smackover brines and associated solids, southern Arkansas

The isotopic composition of Sr has been measured in brine samples from the Upper Jurassic Smackover Formation in southern Arkansas; 87Sr 86Sr ratios range from 0.7071 to 0.7101. With one exception, the 32 Smackover brines contain Sr which is significantly more radiogenic than the Sr in Late Jurassic sea water, indicating sizable Sr contributions from detrital sources. Isotopic analyses of core samples from rock units associated with the brines and regional stratigraphic relationships suggest that the radiogenic Sr was released from detrital minerals in Bossier shale to interstitial fluids expelled from the underlying Louann Salt in the North Louisiana salt basin. These fluids migrated through the Bossier Formation updip to the South Arkansas shelf, where they entered the upper Smackover carbonate grainstone. The radiogenic fluids mixed with Sr-rich interstitial marine waters that had the isotopic composition of Late Jurassic sea water; mixing in variable proportions resulted in the random distribution pattern of variable 87Sr 86Sr ratios that is observed in Smackover brines within the 5000 km 2 study area. Isotopic analyses of nonskeletal carbonate grains and coexisting coarse calcspar cement from the upper Smackover grainstone imply that the grains were diagenetically stabilized in the presence of interstitial marine waters, whereas the calcspar cement is a relatively late diagenetic phase precipitated after the arrival of radiogenic fluids.

ENVIRONMENT RECONSTRUCTION FOR A PART OF THE GLEN ROSE LIMESTONE, CENTRAL TEXAS

SUMMARYAn approximately 50 ft. stratigraphic section of the Lower Cretaceous Glen Rose Limestone was sampled at 24 separate localities in central Texas from the standpoint of reconstructing the depositional environment. Among these samples 199 representative specimens were selected and subjected to point‐count analyses, X‐ray analyses, and insoluble residue determinations. Statistical analysis of the accumulated data with an IBM 7090 computer yielded the following facies: Corbula facies—characterized by a relative abundance of thick‐shelled ostracods, thin‐walled miliolid foraminifers, and the small pelecypod Corbula martinae; steinkern facies—characterized by large mollusc steinkerns, cellular mollusc shells and the foraminifer Orbitolina; mudstone facies—typified by less than 10% sand‐sized grains; and mixed particle facies—characterized by worn skeletal and nonskeletal carbonate grains. The mud‐stone facies subsequently was divided into two subfacies: (1) lime mudstones—characterized by delicate skeletal constituents and lime mud, and (2) marly mud‐stones—consisting of a mixture of lime mud and terrigenous clay‐sized material. Similarly the mixed particle facies was divided into four subfacies: skeletal calcarenites—characterized by sand‐sized skeletal debris; skeletal wackestones—consisting of sand‐sized skeletal particles floating in a mud matrix; nonskeletal calcarenites—characterized by nonskeletal carbonate grains; and nonskeletal wackestones—consisting of nonskeletal carbonate grains floating in a mud matrix. In addition limey sandstone, dolomite, and stromatolite facies were distinguished on the basis of relatively obvious textures and compositions.The attributes of these facies as evidenced by the 199 statistically analyzed specimens were then used to assign each of the additional samples (350) to a particular facies and to identify the distribution of these facies in the field. A reconstruction of the depositional environment was made for each facies, and the following depositional history was interpreted from the resulting facies pattern.The lowermost beds of the unit, consisting largely of stromatolites and nonskeletal calcarenites, are interpreted as representing deposition in very shallow, probably intertidal, waters, Following the deposition of these beds, the depth of water increased