Zakaria Lasemi

Transformation of aragonite-dominated lime muds to microcrystalline limestones

Scanning electron microscopy of textures in several well-lithified Pleistocene “micrites” (mixed micrite and microspar) from south Florida and the Bahamas shows significant diagenetic alteration of the lime-mud precursors. The dominance of aragonite in those precursor muds is supported by the surprising presence of abundant aragonite relics in the neomorphic calcite crystals (confirmed by Feigl stain, X-ray and electron diffraction). Polished, very lightly etched surfaces of neomorphic calcite crystals, even in the microspar, show aragonite relics and pits, both absent in coexisting void-fill cements. Similarities between micrite and microspar crystals indicate their common origin through a one-step neomorphic process of calcitization. That is, the mud did not first calcitize to micrite and then alter to microspar by aggrading neomorphism. This conclusion has more general implications for the interpretation of the origins of other, older microsparitic carbonates. Both early calcitization and concurrent early meteoric cementation were significant in producing the Florida-Bahamas “micrites” studied, which have low porosities similar to those found in most ancient fine-grained limestones. These low porosities, despite the absence of significant overburden, indicate that major porosity reduction without compaction can occur in calcitized aragonite-dominated muds.

Microfabric and Compositional Clues to Dominant Mud Mineralogy of Micrite Precursors

In an earlier study based mainly on a micrite layer in the Pleistocene Miami Oolite in southern Florida, Lasemi and Sandberg (1984) found that microfabric and composition in micrites may be related to the original mineralogy of the precursor lime mud. This chapter takes a broader look at Cenozoic (mainly Plio-Pleistocene) micritic limestones to evaluate the role of original mineralogy in the evolution of microfabric and composition in fine-grained limestones. A number of shallow marine micrites were examined from core or outcrop samples (southern Florida, the Bahamas, and Yucatan, Mexico). Microfabric and compositional data for these samples studied revealed two distinct micrite types (Table 13.1). These types can be interpreted as alteration products of aragonite-dominated precursors (hence, ADP micrites) or calcite-dominated precursors (hence, CDP micrites). The diagenetic microfabrics of monomineralic allochems of known aragonite or HMC (high-magnesium calcite) mineralogy provide strong evidence supporting the inference of mineralogic control on the generation of ADP or CDP micrite microfabrics. The relationship between strontium contents and crystal size in micrites refutes interpretations of aggrading neomorphism of the coarser (“microsparitic”) ADP micrites from the finer CDP micrites. The relatively low strontium contents of CDP micrites also suggest that calcites in the mud precursor were mainly HMC. Taken together, these Cenozoic ADP and CDP micrites can serve as guides for the recognition of original mineralogies in lime mud precursors of ancient fine-grained limestones.

Cement Origin of Supratidal Dolomite, Andros Island, Bahamas

ABSTRACT Dolomite in supratidal carbonates of northwest Andros Island, Bahamas, can be seen, by means of scanning electron microscope (SEM), to be cement in primary porosity and not replacement of the carbonate sediments, as one widely held view suggests. These microcrystalline, calcian dolomite cements commonly occur in lithified lime mud crusts, but they also occur in unconsolidated sediments, mainly as intragranular precipitates. The voids in which the dolomite occurs vary in size from minute wall pores in rotaliid forams and endolithic borings to major intra- and interparticle pores. In the larger pores, dolomite commonly forms an isopachous fringe and may be followed by either aragonite or high-magnesium calcite (HMC) cement. Silt-sized bioclasts and pellets within crusts are often lithif ed by dolomite cement, which may totally fill much of the interparticle porosity. Within unpelleted lime mud, the interparticle porosity may also be filled by dolomite, which engulfs the needle mud particles. The needle mud inclusions or molds visible on polished, etched sections of this dolomite-filled mud are equivalent in abundance, size, and shape to the mud particles in non-dolomitic, unlithified muds embedded in plastic. That equivalence indicates that dolomite replacement of mud has not occurred. Framboidal pyrite present in a few samples appears to have formed prior to or contemporaneously with dolomite cement. The presence of pyrite and the depleted 13C values (-3.2 to -3.9 per mil) of dolomite-cemented crusts sug est that sulfate reduction may have had a role in the formation of Andros Island dolomite cement. Other Holocene and older dolomites from similar settings should be reexamined by means of SEM to determine if they formed partly or wholly as a cement rather than by replacement.

Ancient Carbonate Tidalites

Carbonate tidalites are sediments deposited in supratidal, intertidal and the adjacent shallow subtidal environments by tidal, biogenic, chemical and diagenetic processes and are among the most common deposits in ancient carbonate platform successions. This chapter illustrates sedimentary facies, environments of deposition, and stratigraphy of carbonate tidalites and describes a few analogs from ancient deposits that commonly are encountered in the geological record. Ancient carbonate tidalites consist of a variety of constituents and diagnostic features formed during deposition and early diagenesis in different environments of a tidal system. Peritidal facies are arranged into meter-scale, commonly shallowing-upward succession of subtidal- to tidal flat facies known as parasequence, and may constitute the bulk of the transgressive and highstand packages of a depositional sequence. The geological record of ancient carbonate tidalites indicates deposition in the proximal areas of a tropical sea, particularly during global relative sea level highstands, in carbonate platforms and environments that have recurred many times since the Paleoproterozoic.

New microtextural criterion for differentiation of compaction and early cementation in fine-grained limestones

Mechanisms of porosity reduction in fine-grained limestones (micrites), the most abundant type of limestone, have been difficult to evaluate because of the fine crystal sizes. Scanning electron microscopy reveals common minute voids termed microfenestrae ({approximately} 1.5-40{mu}m diameter) in Holocene lime muds from all marine carbonate environments studied. Experimental compaction of lime muds greatly reduces abundance of microfenestrae at pressures less than 10 bar. Thus, the abundance of cement-filled microfenestrae in micrites appears to be a widely applicable criterion for recognition of the extent of cementation prior to significant compaction in any given micrite. Microfenestrae in most Phanerozoic micrites studied are about as common as in Holocene lime muds, suggesting that early cementation is a more important mechanism in micrite porosity reduction than recent views have suggested.

Depositional facies and sequence stratigraphy of a Lower Carboniferous bryozoan-crinoidal carbonate ramp in the Illinois Basin, mid-continent USA

Abstract The Lower Carboniferous Fort Payne and Ullin Formations in the Illinois Basin form the transgressive and highstand systems tracts that were deposited in a carbonate ramp setting. During deposition of the Ullin Limestone, biotic communities dominated by fenestrate bryozoans and echinoderms (primarily crinoids) proliferated, possibly in response to global tectonic, biological, and oceanographic events that affected bathymetry and nutrient supply. The Fort Payne Formation consists of a dark grey-brown, siliceous and argillaceous lime mudstone in the lower part (transgressive systems tract) and a very fine-grained wackestone to packstone with rare mud mounds in the upper part (early highstand), and was deposited in an outer ramp to basinal environment. During deposition of the lower Ullin Limestone (mostly early highstand), bryozoan-crinoidal build-ups accreted both laterally and vertically into several relatively large carbonate banks, which were partly surrounded by siliceous Fort Payne sea. Bryozoans (primarily fenestrates) were especially prevalent during the late stage of bank development and formed mud-free bioherms up to 120 m thick. In places, carbonate mud mounds also formed during the early stage of bank deposition. Bioherm development declined during deposition of the upper Ullin Limestone (late highstand), and a broad, storm-dominated carbonate ramp was established that became the site for widespread deposition of bryozoan-crinoidal sandwaves. Gradual shallowing led to ooid formation at the end of Ullin deposition. This sequence was terminated by a relative rise in sea level that resulted in deposition of the transgressive facies of the lower part of the overlying Salem Limestone. The depositional style and the nature of skeletal material of the Fort Payne and Ullin Formations are similar to those of cool-water carbonates. A deep-water setting along with upwelling of cool, nutrient-rich oceanic waters may have been responsible for the proliferation of bryozoans and crinoids at this time. The deep-water setting and abundant nutrient supply also may have restricted the formation of ooids and proliferation of shallow-water calcareous organisms.

Recognition of Original Mineralogy in Micrites: ABSTRACT

Detailed SEM study of selected micrites (< 4 µm) and microspars (4 to 12 µm) from all Phanerozoic systems and various geographic localities suggest that textural properties of micrites and microspars are mineralogically controlled. Those micrites and microspars with apparent aragonite-dominated lime mud precursors (ADP) have neomorphic calcite crystals which show pitted surfaces or relic aragonite inclusions in polished, etched sections. The presence of relics in all crystal sizes in ADP micrites and microspars indicates an absence of secondary dissolution-precipitation or aggrading neomorphism. That is, formation of all neomorphic crystal sizes occurred in a single diagenetic event. Micritic limestones with apparent calcite-dominated precursors (CDP), howeve , are characterized by finely crystalline (< 4 µm) textures, lack any inclusions, and have unpitted crystal surfaces. Strontium content of micrites and microspars studied are bimodally distributed. A similar distribution was recognized by Veizer in 1977 and Veizer and Demovic in 1973 and 1974, who suggested it was the result of original mineralogy. Preliminary results on the micrites and microspars studied show Sr distribution generally well correlated with textural properties. ADP and CDP micrites and microspars possess Sr values which fall, with few exceptions, within the high-Sr and low-Sr groups, respectively, of Veizer and others. Several ADP samples which fall within the low-Sr group are molluscan-rich. Thus, likely aragonite contribution to precursor muds was more probably low-Sr. Low-Sr ADP could also be the result of open-system diagenetic alteration. In such cases, low-Sr ADP micrites and m crospars are associated with high Mn content. We have found several low-Sr ADP samples that are high in Mn. High-Sr ADP micrites and microspars are, therefore, interpreted as being originally composed of high-Sr aragonite mineralogy. The Sr content of CDP End_Page 499------------------------------ micrites studied thus far is low and suggestive of high-Mg calcite mud precursors for those micrites. Oxygen isotopic composition of both CDP and ADP samples are rather broad ranges suggesting varying contributions of original mineralogies for both groups. There is, however, an overall negative trend toward lighter ^dgr18O isotopic values with increasing age, indicative of either progressively higher temperatures with age or lower 18O16O ratios in ocean water. End_of_Article - Last_Page 500------------