Carbonate sedimentology: An evolved discipline

Carbonate mud is a major constituent of recent marine carbonate sediments and of ancient limestones, which contain unique records of changes in ocean chemistry and climate shifts in the geological past. However, the origin of carbonate mud is controversial and often problematic to resolve. Here we show that tropical marine fish produce and excrete various forms of precipitated (nonskeletal) calcium carbonate from their guts ("low" and "high" Mg-calcite and aragonite), but that very fine-grained (mostly < 2 μm) high Mg-calcite crystallites (i.e., > 4 mole % MgCO(3)) are their dominant excretory product. Crystallites from fish are morphologically diverse and species-specific, but all are unique relative to previously known biogenic and abiotic sources of carbonate within open marine systems. Using site specific fish biomass and carbonate excretion rate data we estimate that fish produce ∼6.1 × 10(6) kg CaCO(3)/year across the Bahamian archipelago, all as mud-grade (the < 63 μm fraction) carbonate and thus as a potential sediment constituent. Estimated contributions from fish to total carbonate mud production average ∼14% overall, and exceed 70% in specific habitats. Critically, we also document the widespread presence of these distinctive fish-derived carbonates in the finest sediment fractions from all habitat types in the Bahamas, demonstrating that these carbonates have direct relevance to contemporary carbonate sediment budgets. Fish thus represent a hitherto unrecognized but significant source of fine-grained carbonate sediment, the discovery of which has direct application to the conceptual ideas of how marine carbonate factories function both today and in the past.

Flume experiments with fine-grained carbonate particles (< 62.5 µm) show that they form floccules that travel in bedload, form current ripples, and deposit laminated sediments. In order to compare our flume experiments to work on sand transport, we ran flume experiments with medium sand and observed that the velocities at which sand grains started to move and form ripples were in the same range as those where floccules move and form ripples. These ripples are in essence identical to those formed by clay-mineral floccules or sand grains under similar conditions. In light of previous experiments with clay minerals, these results indicate that, of the key controls on mud deposition, flocculation and suspended-sediment concentration are more important than particle mineralogy or water chemistry, and are about as important as bottom shear stress for deposition. Suspensions of carbonate mud show the same pattern of flocculation, ripple formation, and bed accretion as observed previously in experiments with clay-mineral suspensions. The resulting carbonate mud deposits show internal low-angle (2–5 degrees) laminae, and in plan view a pattern of ripple foresets that is identical to rib-and-furrow structure in sandstones. Just as previously assumed for terrigenous muds, there has been a long-standing notion that accumulation of abundant carbonate mud reflects quiescent conditions of offshore and deeper-water environments. These experiments demonstrate unequivocally that carbonate muds can also accumulate in energetic settings. In the sedimentary record of carbonate rocks, interbedded grainstones and lime mudstones may thus not necessarily reflect shifts in depositional energy (or water depth), but alternatively may imply a shift in supplied sediment type. The observations we report suggest that published interpretations of ancient lime muds and derived paleoceanographic conditions may need to be reevaluated.

The landslide problem

ABSTRACTCarbonate hardgrounds often occur at the surface of shallow subtidal to supratidal, lacustrine, and subaerial carbonate shelf sediments. These are commonly disrupted and brecciated when the surface area of these crusts increases. In the subtidal environment, megapolygons form when cementation of the matrix causes the surface area of the hardgrounds to expand. Similar megapolygons form in the supratidal, lacustrine and subaerial settings when repeated incremental fracturing and fracture fill by sediment and/or cement also causes the area of the hardgrounds to expand. The arched up antiform margins of expansion megapolygons are known as tepees.The types of tepees found in the geological record include:(1) Submarine tepees which form in shallow carbonate‐saturated waters where fractured and bedded marine grainstones are bound by isopachous marine‐phreatic acicular and micritic cements. The surfaces of these brecciated crusts have undergone diagenesis and are bored. Unlike tepees listed below they contain no vadose pisolites or gravity cements;(2) Peritidal and lacustrine tepees are formed of crusts characterized by fenestral. pisolitic and laminar algal fabrics. This similarity in fabric makes these tepees of different origins difficult to separate. Peritidal tepees occur where the marine phreatic lens is close to the sediment surface and the climate is tropical. They are associated with fractured and bedded tidal flat carbonates. Their fracture fills contain geopetal asymmetric travertines of marine‐vadose origin and/or marine phreatic travertines and/or Terra rossa sediments. The senile form of these peritidal tepees are cut by labyrinthic dissolution cavities filled by the same material. Lacustrine tepees form in the margins of shallow salinas where periodic groundwater resurgence is common. They include groundwater tepees which form over evaporitic ‘boxwork’ carbonates, and extrusion tepees which also form where periodic groundwater resurgence occurs at the margins of shallow salinas, but the dominant sediment type is carbonate mud. These latter tepee crusts are coated and crosscut by laminated micrite; the laminae extend from the fractures downward into the underlying dolomitic micrite below the crust.Both peritidal and lacustrine tepees form where crusts experience alternating phreatic and vadose conditions, in time intervals of days to years. Cement morphologies reflect this and the crusts often contain gravitational, meniscus vadose cements as well as phreatic isopachous cement rinds.(3) Caliche tepees which are developed within soil profiles in a continental setting. They are formed by laminar crusts which contain pisolites, and fractures filled by micritic laminae, microspar, spar and Terra rossa. Most of the cements are gravitational and/or meniscoid.In ancient carbonates, when their cementation and diagenetic fabric can be interpreted, tepee structures can be used as environmental indicators. They can also be used to trace the evolution of the depositional and hydrological setting.

Sediments recovered from the upper-slope and outer-shelf Great Barrier Reef transect (Ocean Drilling Program Leg 133, Sites 819 through 821) contain a high-resolutio n, 1.5-m.y. record of mixed siliciclastic and carbonate sedimentation. More than 30 multimeter-scale couplets of fining- and coarsening-upward sediments having varying proportions of carbonate and authigenic glauconite are present. These couplets stack into cyclic bundles observed in carbonate and magnetic susceptibility data that, in turn, vary systematically with respect to systems tract development deduced from sequence-stratigraphic analysis. At Site 821, these bundles and their variations clearly track relative changes in sea level for the past ~ 1.48 m.y. The data presented suggest that these various levels of cyclicity are expressed as the result of high sedimentation rates along the margin and that the dominant control on their development has been climate variations that modulated variations in sea level and terrigenous influx. The average duration of many of these cycles is in the Milankovitch waveband and is related to both fourth- (102-443 ka) and fifth-order (34-88 ka) cycles of sea-level change. These cycles are examined with respect to their lithology (color, percentage of total foraminifers, bioclasts, siliciclastics, authigenic glauconite, nannofossils, and tunicates), sequence stratigraphy, downhole log characteristics, magnetic susceptibility, and percentage of carbonate. Time series and Fourier transform results from high-resolution magnetic susceptibility and sonic velocity data suggest strong orbital forcing of the sedimentary cycles, although with differing intensities for different depositional packages and time frames. Magnetic susceptibility/carbonate cycles are interpreted as pulses of deposition of fine-grained siliciclastics during times of maximum flooding (along downlapped surfaces) and subsequent highstand progradation. These pulses dilute carbonate and contain fine-grained magnetically susceptible detrital minerals.

Holocene shallow-water carbonate and evaporite sediments are forming in the Khor al Bazam, a saline lagoon on the coast of Abu Dhabi, southwest Persian Gulf. The sediments are composed of (1) skeletal grains, including whole and fragmented mollusks, corals, calcareous algae, bryozoans, foraminifers, and ostracods; (2) nonskeletal grains, including oölites, pellets, and pellet aggregates; (3) carbonate mud; (4) noncarbonate minerals including gypsum, anhydrite, and terrigenous quartz; and (5) organic material incorporated within mud and carbonate grains. Lithofacies are (1) coral and coralline-algae facies, (2) oölitic sand facies, (3) pellet-aggregate and pellet facies, (4) mud and pellet facies, (5) molluscan sand facies, (6) algal mat facies, and (7) evaporite facies. Nonskeletal calcium carbonate sediment types are related to wave energy; oölites form in the most turbulent environments, pellet aggregates in moderately sheltered environments, and pellets and muds in areas of low wave energy. Skeletal material is altered by blue-green algae. The grains contain algal cells and threads invested in mucilage. Respiration and photosynthesis by algae dissolve the original shell material and reprecipitate microcrystalline aragonite. Chemical changes within the investing organic material cause blackening of some of the carbonate grains.

Bioerosion and carbonate mud production on high-latitude shelves

Based on detailed analyses of cores covering the lagoon of Rasdhoo Atoll, Maldives, six carbonate facies, one soil, and one peat facies have been identified. The abundance of carbonate and rare opaque grains was quantified with a point counter. X-ray diffractometry was used to measure mineralogical composition of samples. The statistical delineation of facies using cluster analysis was based on point count, mineralogical, and textural analyses. In decreasing abundance, the six carbonate facies are classified as mollusk–coral–algal floatstone to rudstone (30 %), mollusk–coral–red algae rudstone (23 %), mollusk–coral–algal wackestone to floatstone (23 %), mollusk–coral wackestone (13 %), mollusk–coral mudstone to wackestone (9 %), and mollusk mudstone (2 %). The carbonate facies represent lagoonal background sedimentation, mostly consisting of fine sediments, and event sedimentation depositing transported coarse-grained reefal components. Fifty-seven carbonate samples and one peat sample were dated radiometrically, covering the Holocene transgression from 10 kyrs BP until today. Comparing the sediment accumulation data of the lagoon with two local sea-level curves, three systems tracts can be identified: (1) a lowstand systems tract characterized by karst and soil deposition >10 kyrs BP, (2) a transgressive systems tract with peat and carbonate separated by hiatus 10–6.5 kyrs BP, and (3) a highstand systems tract dominated by carbonate sedimentation 6.5–0 kyrs BP and further divided into three stages (6.5–3, 3–1, and 1–0 kyrs BP). During the Holocene transgression, sedimentation rates increased continuously to a maximum of 1.4 m/kyr during 3–1 kyrs BP. Modern (1–0 kyrs BP) mean sedimentation rates average 0.6 m/kyr. A simple calculation suggests that two processes (background sedimentation and sand apron progradation) will probably fill up the accommodation space of the lagoon during the Holocene highstand, but these processes will not suffice to fill the larger atoll lagoons of the archipelago.

The responses to attrition and redistribution of the skeletal elements of the major sediment-contributing taxa govern interpretation of the depositional environments of skeletal carbonate sediments. The sediments, biotas, and physical conditions of the Bermuda platform provide data for such an integrated interpretation. Inward from the margin of the Bermuda carbonate platform there is a succession of environments: (1) a reef-front terrace; (2) a reef tract; and (3) a shallow central lagoon. The southwestern half of the lagoon is characterized by reef flats and clusters of patch reefs; these subdivide the lagoon into small basins. The northeastern half occupies a single large basin. The beaches of the Bermuda Islands, a series of Pleistocene dune ridges on the southern edge of the platform, constitute a fourth environment. Carbonate muds are found on deeper bottoms in the more protected basins, molluscan-algal sands in the less protected basins and at the shallower depths, and coralline-foraminiferal-algal sands in the reef tract and reef-front terrace. Trend-surface analysis of sedimentary and biotic data (based on approximately 300 grab samples and supported by direct observations made by snorkeling and SCUBA diving), stratified according to depth, indicates a clear correlation with factors in the physical environment. The textural parameters are related to those organisms contributing to the and are related to the depositional environment by discriminant function and factor analysis. Artificial sediment obtained from the breakdown of hard parts of assemblages of Bermuda organisms in laboratory tumbling-barrel experiments compare closely with sediments on the platform. The sedimentary record is a product of (1) the ecologically controlled distribution of organisms, (2) the size distributions resulting from the breakdown of the hard parts of the major contributors, (3) the sorting and transport of the skeletal detritus, and (4) the degree of mixing of sediments derived from different environments. End_of_Article - Last_Page 553------------

Large concentrically laminated carbonate grains (here referred to as pisoids) have been observed sporadically throughout the geological record and in modern environments. Explanations for how these grains form have varied widely in different settings, although microbial effects are often involved. In Ore Lake, a ~1 km2 flow-through lake in southeast Michigan, one to four centimeter oblong calcite pisoids are observed in both lake bottom shallows and mounded as a small spit near the primary outflow. In section mm-scale light and more porous along with and dark and more dense concentric laminations are apparent. Here we use field observations, petrography, water chemistry, and stable isotopes to understand their formation. Measurements of pisoid calcite δ18O and lake water δ18O indicate that precipitation occurs in waters between roughly 19 – 28°C. These warm temperatures imply that pisoid growth happens almost entirely within the summer, contrary to prior work that suggested wintertime precipitation was important. Pisoid δ18O values largely overlap with coexisting lake bivalve values, suggesting that pisoid precipitation is in equilibrium. In contrast, pisoid δ13C is as much as 8‰ more positive than bivalve δ13C due to photosynthetic effects. We propose that the laminations in these pisoids arise from different rates of formation within the warm months, rather than large seasonal differences. A decline in lake alkalinity beginning in late spring likely coincides with more rapid growth, with slower growth mediated by cyanobacteria continuing through the summer. This range of observations enables the use of Ore Lake as a potential model for understanding pisoid formation throughout the geological record.

Lunar mineralogy

The Upper Jurassic Hadriya and Hanifa carbonate grainstone reservoirs in Berri Field, Saudi Arabia, contain large reserves of Saudi Extra Light crude oil. New geologic models of each reservoir have been built using sequence-stratigraphic principles. These new models have a chronostratigraphic architecture which accommodates the facies transitions observed in cores sampled along the ramp-to-basin profile. Each reservoir contains, from oldest to youngest:(l) a highstand systems tract represented by aggrading and prograding grainstones, capped by a type-2 (subaqueous) sequence boundary; (2) a shelf margin wedge systems tract characteriz.ed by grainstones showing maximum basinward progradation; and (3) a transgressive systems tract typified by a series of backstepping grainstones capped by a surface of relative drowning.These new models clarify the elements controlling fluid movement within the reservoirs.

Facies relationships in abandoned Holocene Mississippi Delta complexes are characteristic of both retrogradational transgressive systems tracts (TST) and progradational highstand systems tracts (HST). In the Barataria interlobe basin, delta-plain facies of the early Holocene Maringouin/Teche delta complex (TST), which accumulated from 7500 to 6000 yr BP, are overlain by a lagoonal facies 1-2 m thick (MFS) that accumulated during the maximum flooding event from 6000 to 3500 yr BP. Wave reworking transformed the distributary sands of retrogradational delta complexes into stratigraphically backstepping shoreline sand bodies. The most landward of these shorelines, the Teche shoreline, overlies the MFS and is, by definition, the shoreline of maximum transgression (SMT) (Penland et al. 1987 , 1987b; Kosters 1989). Relatively thick peats of high organic content, dating from 2400 to 1100 yr BP, are located immediately landward of this shoreline. Younger delta lobes, rapidly prograding since 1100 yr BP, have shifted the coastline seaward of the Teche shoreline, and form the first progradational HST parasequence. Thin, organic-poor salt marsh sediments are accumulating within this parasequence landward of the present shoreline. Rising relative sea level provides increased accommodation space while fresh water may be held within the delta plain, creating conditions of both groundwater and nutrients favorable to accumulation of high-quality organic facies of this type. In a subsequent progradational setting, stable relative sea level results in less accommodation space landward of the shoreline, while fresh water and nutrients are discharged into the Gulf of Mexico, forcing formation of brackish and salt marsh environments, unfavorable to accumulation of high-quality organic facies. These hypotheses may help explain the variability of some littoral high-quality coals vs. carbonaceous shales in the rock record.

The rift section of the Brazilian basins represent the sedimentary record associated with the first stages of Gondwana break-up in the Early Cretaceous phase (Berriasian to Aptian). The rift succession of the Campos Basin constitutes one of the main petroleum systems of Brazil’s marginal basins. This interval contains the main source rock and important reservoirs in the Lagoa Feia Group deposits. The Lagoa Feia Group is characterized by siliciclastic, carbonate and evaporite sediments deposited during the rift and post-rift phases. Despite the economic relevance, little is known in stratigraphic terms regarding this rift interval. To date, most studies of the Lagoa Feia Group have adopted a lithostratigraphic approach, while this study proposes a tectonostratigraphic framework for the deep-rift succession of the Campos Basin (Lagoa Feia Group), using the fundamentals of seismic sequence stratigraphy. This work also aims to establish a methodological and practical procedure for the stratigraphic analysis of rift basins, using seismic data and seismofacies, and focusing on tectonicstratigraphic analysis. The dataset comprised 2D seismic lines, core and lithological logs from exploration wells. Three seismic facies were identified based on reflector patterns and lithologic data from well cores, providing an improved subdivision of the pre-, syn- and post-rift stages. The syn-rift stage was further subdivided based on the geometric patterns of the reflectors. Tectonics was the main controlling factor in the sedimentary succession, and the pattern and geometry of the seismic reflectors of the syn-rift interval in the Campos Basin allowed the identification of three tectonic systems tracts: (i) a Rift Initiation Systems Tract; (ii) a High Tectonic Activity Systems Tract and (iii) a Low Tectonic Activity Systems Tract.

The Ca Voi Xanh (CVX) gas field is located offshore Vietnam along the eastern margin of the southern Song Hong Basin. Reservoir rocks are carbonates of Middle Miocene (Langhian and Serravallian) age which developed on an isolated platform (length approximately 100 km, and width approximately 15 km) on top of the Triton Horst structural high. Shallow‐water corals and large and small benthic foraminifera are the main faunal constituents of the Langhian carbonates, whereas overlying Serravallian carbonates, the principal reservoir at CVX, are dominated by deeper‐water coralline red algae (rhodoliths) and large benthic foraminifera (LBF). The Serravallian carbonates consist of rhodolith‐LBF grainstones and packstones to mud‐lean packstones. Langhian carbonates consist of coral‐LBF grainstones‐packstone/rudstones.This paper documents the workflow used to develop an integrated sequence‐stratigraphic and reservoir rock‐type framework for the Ca Voi Xanh reservoir, and the impact of this work on reservoir quality prediction and modelling. Sequence stratigraphic interpretations of the Serravallian carbonates are based on data from three wells including detailed sedimentological core descriptions tied to well‐log character. The carbonate succession consists of three third‐order depositional sequences (Ser1, Ser2 and Ser3). Two well‐developed exposure surfaces can be identified: the base‐Serravallian sequence boundary (Ser1 SB), and the base‐Tortonian sequence boundary (Tor1 SB). The Serravallian shows an overall shallowing‐upward trend from carbonates with more horizontally‐oriented coralline red algae (encrusted and bored pavements/hardgrounds) in the lower part of the section, to carbonates with large, roundish irregular rhodoliths towards the upper part. Petrographic thin section, stable isotope (oxygen and carbon), and fluid inclusion analyses confirm a freshwater (vadose and phreatic) diagenetic overprint of the carbonates below the exposure surfaces (sequence boundaries).Partial dolomitization of the Serravallian carbonates was observed at Wells‐2 and ‐3 in the CaVoi Xanh field. Dolomite formation could be related to hydrodynamic fluid interactions associated with the development of a freshwater lense during the top‐Serravallian sea‐level fall (Tor1 SB), favoring dolomitization by sea water during the subsequent sea‐level rise. Alternatively, dolomitization could be related to CO2 and hydrocarbon charging, causing secondary leaching of the Mg‐rich coralline red algae and large benthic foraminifera, leading to an enrichment of the water with Mg‐ions and thus favoring downward dolomitization.A reservoir rock type (RRT) scheme was developed for the Langhian and Serravallian carbonates at the Ca Voi Xanh field based on a combination of depositional environment interpretations together with diagenetic characteristics and reservoir parameters (porosity and permeability).The Langhian is characterized by two RRTs depending on the degree of cementation (RRT‐3L) and dissolution (RRT‐5L). Serravallian RRTs are separated into dominantly packstone (RRT‐1) and dominantly grainstone textures (RRTs‐2, ‐3, ‐5 and ‐6). RRTs with grainstone texture show varying degrees of cementation (RRTs‐2 and ‐3) and dissolution (RRTs‐5 and ‐6). Early diagenetic dolomitization (RRT‐4) preferentially affects packstones (RRT‐1) but also grainstones (RRT‐2, ‐3 and‐5).The geologic (static) model consists of both matrix and non‐matrix components. Sequence stratigraphic concepts in combination with seismically‐derived palaeo‐reconstructions were used to guide the vertical and lateral distribution of reservoir rock types, which comprise the matrix component of the geological model. Different karst geometries and features consistent with known hydrogeologic processes were identified in the seismic discontinuity (variance) cube, and were used to interpret karst regions with different degrees (and/or types) of karst/fractures. These karst regions were used to populate the non‐matrix component of the geologic model along the Ser1 and Tor1 sequence boundaries.