Patrick F. Rush

Freshwater phreatic diagenesis in the marine realm of Recent Arabian Gulf carbonates

ABSTRACT Analyses of a large suite of intercalated carbonate and terrigenous samples from borings in the Arabian Gulf near A1 Jubayl, Saudi Arabia, reveal complex depositional and diagenetic patterns. The constituents composing the sediments and rocks are similar to those being deposited today in the Arabian Gulf. The most abundant allochems include molluscs, echinoderms, corals, red algae, pellets, ooids, and intraclasts. Terrigenous material ranges from trace amounts comprising the dominant framework constituent. The deposit is composed of three diagenetic facies. The lowermost part of the accumulation is a dolomite (at least 56 m thick) overlain by carbonates composed of low-magnesian calcite (5-23 m thick), while the uppermost portion is dominantly aragonitic in composition (averaging 5 m in thickness). The dolomite facies consists of sediments that accumulated in shallow-water to tidal-flat depositional environments. Dolomitization of the upper part of this facies took place as a result of evaporative processes during subaerial exposure. The nearly ideal stoichiometry, well-ordered structure, and relatively light 18O composition indicates that the dolomites have undergone diagenesis since their formation. The low-magnesian ca cite facies consists of shallow-marine deposits that have undergone diagenesis in a freshwater phreatic zone. These deposits underwent diagenesis (including dissolution of aragonitic allochems and cements, stabilization of high-magnesian calcite allochems, and precipitation of isopachous low-magnesian calcite cements) due to an extensive lens of freshwater that flowed through the sediments while they were beneath the marine waters of the Arabian Gulf. An aragonite-cemented crust acted as an effective upper seal for this freshwater lens. The common isopachous, fibrous aragonite cement indicates that the aragonite facies was deposited and has remained in the shallow-marine realm. Early freshwater phreatic diagenesis is generally interpreted to be associated with near-surface subaerial conditions. The presence of an extensive layer, extending at least 9 km seaward from the strandline, of shallow-marine sediments that have undergone freshwater diagenesis in the phreatic zone while under the marine waters of the Arabian Gulf suggests that previous interpretations of deposits exhibiting freshwater diagenesis may require reevaluation.

Skeletal Mineralogy of Devonian Stromatoporoids

ABSTRACT Previous investigators interpreted stromatoporoids to have been composed originally of aragonite; however, petrographic observations of stromatoporoid microstructure indicate that those within the Devonian Helderberg strata of New York State were originally composed of high-magnesian calcite. The delicate reticulate structure of pillars and growth laminations of the original stromatoporoid coenosteum is represented by a neomorphic fabric of microdolomite-rich zones of inclusions in low-magnesian calcite. In comparison, co-existing constituents that were originally composed of low-magnesian calcite (e.g., brachiopods) display 1) better preservation of their ultrastructure than do the stromatoporoids, and 2) an absence of microdolomite. Gastropods, originally composed of aragonite, are ll represented by micrite envelopes encompassing pore-filling spar accumulations, i.e., complete loss of ultrastructure. The style of preservation of the stromatoporoids is essentially identical to that of associated echinodermal debris, an original high-magnesian constituent. Both the stromatoporoids and echinoderms are preserved as microdolomite-bearing diagenetic low-magnesian calcite. Consequently, these stromatoporoids were originally composed of high-magnesian calcites.

Occult aragonitic fabrrics and structures within microbiolites, Pennsylvanian Panther Seep Formation, San Andres Mountains, New Mexico, U.S.A.

Black microbiolites (stromatolites and oncoids) are common constituents within the 800m thick Pennsylvanian Panther Seep Formation. This formation, which is composed of cyclically repeated sandstones, carbonates, and shales, crops out in south-central New Mexico, U.S.A. The deposits range from those that accumulated within semi-arid to arid tidal flats, cut by small fluvial channels, to evaporitive lagoonal and open shallow marine depositional environments. The microbiolites accumulated predominantly as tidal flat and very shallow subtidal deposits. The black color of these microbiolites is prominent on the outcrop. Unfortunately, on the microscopic scale, the organic matter as well as diagenetic alteration obscures many of the internal structures and fabrics of these organosedimentary features. However, the use of the very simple “white card” technique clearly brings out the occult structures and fabrics and thus permits detailed description of the original fabric of the individual crystals as well as their aggregate morphology. The relict fabric indicates that the microbiolites were originally composed of aragonitic crystals. This interpretation is supported by the high relict Sr content of the present diagenetic lowmagnesian calcitic material. Thus, initially, these microbiolites were essentially totally composed of aragonitic crystals which precipitated perpendicularly to the mucilaginous substrate within the microbiolites.

Diagenetic framework for chemical remanence acquisition in lower Paleozoic carbonate rocks from W. Newfoundland

The Lower Ordovician (Tremadocian) St. George Group on Port au Port Peninsula forms part of the Cambro-Ordovician autochthonous carbonate sequences of W. Newfoundland. The geology and stratigraphy of the St. George Group indicates that these rocks have been subjected to distinct uplift and exposure events and their petrographic characteristics suggest a complex diagenetic history. Paleomagnetic, petrographic, geochemical, and geological evidence will be presented that indicates both Paleozoic components record chemical remanent magnetizations associated with subaerial exposures of the ancient continental shelf during at least two distinct times in the Paleozoic. Given the observed range of petrographic and magnetic variability, it appears that during the (Early-Middle ) Ordovician, the lower St. George rocks underwent relatively rapid diagenesis, including lithification, calcite cementation, sulfide precipitation, dolomitization, dedolomitization, and hematite authigenesis. The Late Paleozoic component appears to represent a remagnetization component also associated with dedolomitization and hematite authigenesis and cementation. Perhaps more important, rock samples bearing hematite as the major remanence phase can be petrographically identical to one another, yet carry distinct directional components acquired at different times in geologic history.