Early Marine Cementation Research Articles

Diagenesis of Lime Mud, Mississippian-Age Bioherms, Sacramento Mountains, New Mexico: ABSTRACT

Samples from six Waulsortian bioherms were examined from the Mississippian Lake Valley Formation, Sacramento Mountains, New Mexico, to determine the timing and mechanisms of lime-mud cementation and to evaluate the role of this cementation in biohermal stabilization and growth. Petrography and cathodoluminescence of bioherm and interbioherm muds defined distinct diagenetic mud types. Each mud type is characterized by a distinct grain size, mud color, and cathodoluminescence. Bioherm type 1 mud is dark gray, fine-grained, End_Page 559------------------------------ (Y = 3.7 µm) and has a dull luminescence. Type 2 mud, which coexists with type 1 mud, is coarse grained (Y = 5.7 µm), lighter gray, and has a brighter luminescence. Type 3 mud is darker gray, coarser-grained (Y = 6.7 µm), and is nonluminescent. Type 4 mud is gray brown, and is the coarsest grained (Y = 8.8 µm) and most brightly luminescent of all bioherm mud. For comparison, interbioherm mud has the finest mean grain size (Y = 3.36 µm and the brightest luminescence of all mud types examined. It is proposed that synsedimentary marine cementation of biohermal muds and within synsedimentary stomatactis is the causative factor for bioherm stabilization and growth. Dark gray nonluminescent bioherm muds reflect a significant component of synsedimentary marine cements while lighter gray luminescent bioherm muds reflect a higher proportion of post-burial intergranular cements. This interpretation is supported by the oxygen and carbon isotopic composition of the lime-mud types. For example, synsedimentary stromatactis cavities lined by fibrous marine cements are constructed from nonluminescent type 3 muds which possess the heaviest isotopic signatures (-1.8 to -2.7 ^pmil^dgr18O; + 4.81 to +4.1 ^pmil^dgr13C). In contrast, cavities infilled by post-burial granu ar cements are associated with luminescent mud types 1, 2, and 4, which possess lighter isotopic signatures (-2.3 to 14.4 ^pmil^dgr18O; +2.0 to +4.2 ^pmil^dgr18O). Interbioherm muds, interpreted to have lithified at a later stage than bioherm muds, possess lighter isotopic signatures (-3.8 to -6.7 ^pmil^dgr18O; +2.3 to +2.5 ^pmil^dgr13C). Because of the heterogeneous distribution of synsedimentary cemented muds within the bioherms, the formation of the stromatactis cavities characteristic of Waulsortian bioherms is interpreted to occur from the slumping and subsequent removal of uncemented grains from beneath patches of early marine cemented mud. As such, this study emphasizes the role of early marine cements in fine-grained sediments as an active process which merits careful consideration in determining original porosities and diagenetic histories in ancient fine-grained carbonate facies. End_of_Article - Last_Page 560------------

Early Carbonate Fabrics in Silurian Reefs of Gotland, Sweden: ABSTRACT

The Hogklint reefs of Gotland occur within a shallowing-upward sequence and are developed in the outer zones of a carbonate wedge adjacent to a shale belt. Shoal and peritidal sequences are the major carbonates which occur in the inner parts of the wedge and are preferentially associated with erosion surfaces. The Hogklint reefs show carbonate fabrics that are inferred to indicate synsedimentary marine cementation of the reefs. These fabrics are: (1) pseudofibrous calcites that are believed to have replaced acicular/fibrous cements with original aragonite and/or Mg-calcite mineralogy, (2) pelmicsparite-pelmicrite areas, (3) stromatolite-like crusts, and (4) early micritic cements. The reefs also show evidence of subaerial exposure by the occurrence of erosion surfaces at the top of the reefs, dissolution cavities, and vadose crystal silt infills in cavities in the algal biofacies which cap the reefs. End_Page 640------------------------------ The distribution of the various cement types within the reefs can be linked with the shallowing-upward nature of the limestone sequence as there is a predominance of early marine cement fabrics in the middle and upper parts of the reefs, particularly in the algal biofacies. The spatial and sequential nature of the various carbonate diagenetic fabrics show that the Hogklint reefs went through four main diagenetic events. Microstalactitic cements, vadose crystal silts, grain-contact cements, and pseudofibrous isopachous cements are developed within cross-bedded peloidal grainstones at the top of the shallowing-upward sequence and indicate cementation within the freshwater vadose zone and intertidal and/or marine phreatic zone. End_of_Article - Last_Page 641------------

Sedimentology and paleoecology of Upper Ordovician mounds of Anticosti Island, Quebec

Relatively thin organic buildups in the Ellis Bay Formation of Anticosti Island developed in a shallow subtidal regressive marine shelf environment during the Late Ordovician. The Ellis Bay Formation has been subdivided into six members by Bolton. Two buildups, one in each of members 4 and 6, were studied in detail. The member 6 mud mound on the Salmon River (8 m thick) is bound by calcareous algae, stromatoporoids, and corals, and consists of a micritic bafflestone core capped by crinoidal lime grainstone. Early marine cementation permeated nearly all of the primary porosity. Cathodoluminescence indicates at least five stages of cementation of the mound. Constructive mound development was terminated by progressively shallowing, agitated marine conditions.The member 4 mound is a small coral (ecological) reef exposed on the Vaureal River.

Diagenesis of Lower Coralline Limestone (Chattian), Maltese Islands: ABSTRACT

The sequence of diagenetic fabrics in the lower Coralline Limestone (Chattian) on the Maltese Islands suggests that there was one phase of early marine cementation followed by at least two periods of phreatic cementation. Cements of marine origin include fine fibrous cement on algal and foraminiferal debris, and clouded syntaxial overgrowths on echinoid fragments. A younger generation of clear overgrowths on echinoid fragments displays luminescent zones that are restricted to the lower Coralline Limestone and lower member of the Globigerina Limestone and can be correlated throughout the Maltese Islands. These clear overgrowths are phreatic and formed both before and during sediment compaction. Subsequent phreatic cementation produced fine to medium-grained, non-luminescin scalenohedral calcite crystals that postdate compaction. The final stage of phreatic cementation consists of fine to medium-grained equant void-filling spar that is non-luminescent. Vertical and lateral distribution of phreatic cements and compacted textures is irregular and discontinuous. In general, well-cemented horizons also show overcompacted textures. The relative timing of these diagenetic features indicates at least two episodes of emergence and meteoric cementation related to the development of freshwater lenses within the lower Coralline Limestone. Erosional and unconformable horizons in the overlying Miocene formations may record times of freshwater alteration corresponding to periods of eustatic lowering of sea level and emergence of the entire central Mediterranean platform. End_of_Article - Last_Page 682------------