Bioclastic Mudstone Research Articles

Organic matter composition and distribution through the Aptian–Albian lacustrine sequences of the Araripe Basin, northeastern Brazil

The main characteristics and distribution of organic matter have been studied through several Aptian–Albian lacustrine units (Rio da Batateira, Crato and Ipubı́ Formations, Araripe Basin, NE Brazil). Six main facies have been distinguished on the basis of their lithological–sedimentological characteristics and the type and amount of organic matter: (1) marginal lacustrine deltaic massive to coarsely laminated mudstones and sandstones found as lenticular sheet and channel-infills; (2) oxic marginal lacustrine massive to poorly laminated mudstones; (3) oxic–suboxic inner lacustrine mudstones and laminated bioclastic mudstones; (4) anoxic, inner lacustrine dark mudstones; (5) anoxic, inner lacustrine dark clay-carbonate rhythmites; and (6) anoxic, inner lacustrine finely laminated dark micritic limestones. These facies make up decimeter- to several meter-thick parasequences, which stack up in several tens of meters thick successions. Water level changes and variations of the detritic sediment contribution in the basin controlled the evolution of the depositional system. Palynomorph associations showed that the lacustrine system evolved in an arid–semiarid tropical area where the most characteristic flora consisted of conifers, Gnetales, Bennettitales and early angiosperms. In facies 1 to 3, organic matter was scarce, poorly preserved and mainly derived from higher land plants. Facies 4 is characterised by containing abundant algal/bacterial-derived well-preserved organic matter. Facies 5 and 6 contained mainly branchlets and pollen debris blown by the wind or fed by lacustrine underflows. The pattern of organic matter accumulation and preservation corresponds well with cyclical palaeoenvironmental changes (from anoxic to oxic bottom conditions and from a fresh to saline upper water layer), which affected the shallow lacustrine system. The deposition of thin, organic matter-rich mudstones (facies 4) probably occurred during higher water levels, which resulted in relatively low salinity waters and low detrital contribution to the inner lacustrine zones. Less significant organic matter accumulation is recorded in the clay-carbonate rhythmites and in the limestones (facies 5 and 6) due to scarce autochthonous aquatic organic matter deposition and pervasive intense bacterial degradation, caused by persistent meromictic conditions. Large differences in the degree and path of evolution of the humic material derived from higher land plants have been detected, depending on the depositional conditions. The humic material in the oxic facies underwent humification and gelification and had the highest huminite reflectances, whereas it showed suppressed reflectance values in the oil shales (facies 4) and in the carbonate-dominated facies. In the latter, extensive bacterial reworking of humic material is also observed. These differences are considered significant and applicable to other case studies in terms of evaluating the maturity of organic matter in ancient lacustrine systems.

Characterization of the Karababa-C reservoir in the South Karakuş oilfield, southeast Turkey

The South Karakuş oil field is located in southeast Turkey, and produces, from the Late Cretaceous Mardin Group, carbonate reservoirs consisting of the Karababa, Derdere, Sabunsuyu and Areban Formations. The Karababa Formation is divided into three members, of which the productive Karababa-C member, discussed here, is composed of shallow marine bioclastic mudstone to wackestone. It is affected by early to late diagenetic processes that caused modification of the original pore system by cementation, dissolution and fracturing. Two petrophysical-petrologic facies can be discriminated by principal component analysis, and linear, multiple and stepwise multiple regression analysis of petrophysical data. These correspond to vuggy-moldic porosity and microporosity dominated reservoirs.

A down-slope upper triassic reef mound: Aflenz limestone, Hochschwab Mountains, Northern Calcareous Alps

A small reef mound occurring within the well-bedded, chert-bearing Norian Aflenz Limestones of the Aflenzer Burgeralm (south eastern Hochschwab Mountains, Styria, Austria) is described with respect to microfacies and paleontological criteria. The mound was formed in a down-slope position near the transition between basinal sediments (Aflenz Limestone, exhibiting a shallowing-wards sequence) and marginal platform carbonates (Dachstein Limestone). The transitional zone is characterized by a change from open-marine pelagic carbonates with allochthonous intercalations at the base to protected platform environments as well as shallow-marine platform slope deposits near the top. The biota of the reef mound (corals, calcisponges, hydrozoans, calcareous algae, foraminifera, microporoblematica) correspond with those of Dachsteinkalk reefs. Three communities can be distinguished according to dominant corals and calcisponges (Thecosmilia-Paradeningeria community,Astraeomorpha community,Paravesicocaulis community). The microfacies of the massive, partly dolomitized and stylolitized reef limestone is characterized by a bioclastic floatstone at the base of the mound, overlain by sponge bafflestone, coral-bafflestone, spongiostromate bindstone, coral-sponge framestone, and sponge-hydrozoan bafflestone, without regular distributional patterns. The bedded limestones below the reef carbonates consist of bioclastic lime mudstones with radiolarians; the overlying bedded limestones are composed of peloidal and bioclastic grainstones and packstones as well as rudstones indicating a termination of the reef growth by a rapid allochthonous sedimentation perhaps caused by slumping or storm effects.

Tectonics and Depositional Environments in Early Pennsylvanian of South-Central New Mexico: ABSTRACT

The Pennsylvanian Magdalena Group unconformably overlies Precambrian to Mississippian strata throughout south-central New Mexico. Regional southward tilting, warping, and erosion resulted in progressively older pre-Pennsylvanian subcrop relations from south to north. Normal marine shelf sedimentation was represented by upward shoaling cycles containing mixed siliciclastics and carbonates. Channels cut into Ordovician strata in the southern Caballo Mountain area suggest that this area was a relatively deep valley or trough during Late Mississippian-Early Pennsylvanian erosion. Marine transgression from the south backfilled the preexisting stream valleys with sequences of packstone, wackestone, shale, and coarse quartzarenite. Topographically higher areas were blanketed with thin beds of laminated bioclastic packstone, grainstone, lime mudstone, and shale. Semirestricted tidal to nearshore environments are reflected in local sections by finely laminated hematitic dolomites, oncolitic wackestones, limestone conglomerates containing reworked Mississippian chert, and terrigenous shales supporting petrified wood float. Depositional onlap farther north resulted in a thinner and younger sequence. The cyclic nature of these deposits reflects a proximity to the shoreline and suggests an abruptly shifting base level, possibly controlled by Early Pennsylvanian glacial eustasy. The time-stratigraphic position of the valley-fill depositional unit may be broadly correlated to other Late Mississippian-Early Pennsylvanian valley-fill sequences recognized in the Delaware basinmore » and Grand Canyon.« less

Silurian reefal buildups: Spiti-Kinnaur, Himachal Himalaya, India

The Silurian sequence in Spiti and Kinnaur was studied at Takche, Gechang, Muth-Shian, Leo and Manchap. At all these localities the sequence commences with an intertidal—near-shore argillo-arenaceous succession. The overlying part at Takche comprises arenaceous dolomite, calcareous sandstone and dolomite. It sporadically contains rugose and tabulate corals deposited in a shallow undathem near the shore. The succession at Gechang is more calcareous and in some parts rich in corals; stromatoporoids occur locally. Here bioclastic wacke/packstones and framestones formed a buildup which was possibly formed in a lagoon. The Muth-Shian area is characterized by bioclastic mudstones, bioclastic wacke/packstones and framestones. Reef builders are corals, stromatoporoids and solenoporoids. These sediments seem to be a lateral equivalent of the coastal arenaceous sequence exposed at Baldar and may represent a fringe reef.

Depositional Environments and Diagenesis of Frobisher-Alida Interval, Madison Group (Mississippian, North Dakota Williston Basin: ABSTRACT

The Frobisher-Alida interval is a well log-marker-defined interval, not lithologically based, which extends across middle and upper Mission Canyon carbonates into lower Charles evaporite from the basin center toward the eastern basin margin. The interval represents a regressive sequence with superimposed minor transgressive pulses. A wide spectra of depositional and diagenetic environments with associated fabrics and porosities are represented in these carbonates. Basin-margin carbonates are dominated by supratidal sediments with lesser amounts of intertidal and subtidal sediments. Major fabric types of important producing zones consist of major developments of hypersaline, and minor development of caliche and vadose, ooid and pisolite wackestones to grainstones. Intercalated with these are fenestral mudstones to laminated dolomudstones, gastropod mudstones, and evaporite mush dolomudstones. In addition, desiccation cracks and minor occurrences of karst breccias and laminated crusts indicate periods of subaerial exposure. Porosities associated with the supratidal-subaerial realm include well-developed interparticle, fenestral, vuggy, and intercrystal types, and in places are filled with clear spar and/or void-filling clear anhydrite cement. > Basinward, subtidal sediments dominate the interval. These consist of cyclic, shallow open-marine bioclastic mudstones to packstones-grainstones and, partly restricted marine burrowed peloidal mudstones to bioclastic wackestones. Upsection, non-cyclic burrowed bioclastic peloidal mudstones to wackestones indicate increasingly restrictive conditions. Interparticle porosity is exhibited in the bioclastic grainstones. Intercrystal porosities are developed from selective dolomitization of burrows and mudstones. End_of_Article - Last_Page 614------------

Regional Distribution of Carbonate Facies, Bombay Offshore Region, India

The Bombay offshore region, on the western continental shelf of India, has reservoir facies generally confined to the carbonate sections developed at different stratigraphic levels of the Cenozoic succession. Sedimentologic analysis of cores and cuttings, aided by regional, almost time-parallel, seismic markers, has revealed the presence of bioclastic mudstone and/or wackestone facies, pelletoidal mudstone and/or wackestone facies, and bioclastic grainstone facies in Bombay High, Bassein, and the shelf-edge areas, respectively. A depositional model, both in time and space, which was developed from facies analysis, has prediction capabilities. An east-west lithofacies cross section illustrates the vertical distribution of reservoir facies and broadly represents a shelf-lagoon to shelf-margin depositional setting. Basin configuration, water energy, and periodic fluctuations in sea level are the dominant factors that controlled the development of reservoir facies.