Chapter 11 - Carbonate Rocks

aragonite

"mother of pearl" lining mollusk shells and pearls; less stable than calcite, can alter into calcite during diagenesis; orthorhombic lattice; Ba and Sr can substitute in for calcium; sometimes the instability can decrease the chances of fossilization

allochemical components

(allochems) any grains of calcium carbonate that, after formation, are transported and deposited as clasts; analogous to rock and mineral fragments in the framework fraction of terrigenous sandstone; ooids and bioclasts

nonskeletal allochems

1. various coated grains (ooids, pisoids, and oncoids) 2. peloids 3. clumped or aggregated grains (lumps, grapestones) 4. limestone clasts (limeclasts) - fragments of pre-existing limestone derived from intrabasinal and extrabasinal sources; these clasts are ripped up, transported, redeposited, and confined within limestone strata

Calcite

CaCO3; soft mineral that readily fizzes in acid, rhombohedral form; Mg, Fe, and Mn can substitute for calcium; organisms make there shells out of this material

dolomite

CaMg(CO3)2 - can form Ankerite if there is enough Ferrous iron; third important carbonate mineral

secondary spar

a replacement after micrite; limestones with grains floating in (not grain-supported) were probably originally composed of grains floating in micrite that has been recrystallized

arenite

a sedimentary clastic rock with a sand grain size between .0625 mm and 2 mm

geologic age importance

a variety of calcium carbonate-secreting organisms have evolved over the geologic past

modern ooids consist of

acicular needles of aragonite; some show tangential orientation with long axes and subparallel with the ooid laminae

coated grains

all share a common spherical shape, but they vary in size and degree of internal organization

micrite (limestone family)

analogous to mudrock; consists principally of microcrystalline calcium carbonate matrix in which are scattered less than 10% allochems

crystalline carbonate

any limestone in which the original depositional texture is unrecognizable

most aggregates

bound together by encrusting organisms

ancient ooids composed of

calcite rather than aragonite, and fibrous crystals typically fan out radially from the grain center; recrystallization and dolomization often obscure the original texture and mineralogy

ooids and pisoids

can be biogenic, inorganic, or both; grow by simple accretion as wave, tidal, and storm currents sweep grain nuclei back and forth in shallow marine seawater supersaturated with dissolved calcium and carbonate

pisoids (or pisolites)

coarser coated grains that are identical to ooids in shape and internal organization

orthochemical components

components that are not transported and deposited as clasts; found precisely where they are formed or have been moved only a short distance; micrite and spar

calcareous ooze

consists of the unconsolidated shells of floating pelagic organisms that thrive in the photic zone that extends from the surface of the water to about 200m, only accumulate above CCD

spar under the microscope

crystal clear without the hazy brownish cast of micrite

The Dunham classification

determines whether the sparry cement is primary cement or a secondary recrystallization of micrite; emphasizes limestone texture especially grain (allochem) packing and the ratio of grains to matrix, allochem type is ignored

depositional setting

dictates paleoecology

extrabasinal clasts

differ in composition, texture, and overall character from the limey material in which they occur

intrabasinal clasts

eroded from the same stratigraphic unit, or even the same stratum, in which they are found now

interstitial pores (grain aggregates)

filled with micrite

carbonates

form chemically and biochemically; dissolved ions carried from source to depositional site in solution eventually precipitate and form solid minerals

Grain aggregates

form when such carbonate particles as ooids and peloids adhere to one another; most are fine- to medium-grained sand-sized masses that are given specific descriptive names: grapestones, lumps, and botryoidal lumps; form where water depths are very shallow and only weak bottom currents are present; conditions permit the growth of thriving surface mats of living microbes that help bind grains together

intraclasts

formed of an interlocking mosaic of calcite or aragonite; intrabasinal detrital carbonate; often experience a history of transport and deposition as clasts

ooids

grains formed of an interlocking mosaic of calcite or aragonite

yesterday's micrite

has become today's spar and vice versa

rudstone

have a grain-supported texture coarser than 2 mm

floatstone

have a matrix-supported texture

ooids (also called ooliths)

have a nucleus (often a skeletal fragment or a small clast of detrital quartz) around which concentric layers of calcium carbonate are wrapped; have a diameter of less than 2 mm; most are the size of very fine sand (from 0.2 to 0.5 mm in diameter)

prefix - Folk

identifies the predominant allochem

modern ooids form

in marine settings at shallow water depths, they are able to form in 15 m, but they usually form at less than 5 m; maximum development occurs at depths of less than 2 m.

suffix - Folk

indicates allochem size as either finer (micrite) or coarser (rudite)

name stem - Folk

indicates whether the interstitial material is spar or micrite

any limestone with a sparry cement

is a sparite even if it started out as a micrite

primary sparry cement

limestones in which all the grains touch others (grain-supported) were originally porous and were later cemented

boundstones (equivalent to Folk's Biolithites)

limestones in which the components have been bound together from origin (such as reef rocks); subdivided into framestone, bindstone and bafflestone

Phanerozoic time

limestones reveal much about the shallow marine ocean-covered areas

skeletal allochems

many are present in limestone reflect age and depositional setting

carbonate compensation depth (CCD)

marks the depth at which slowly falling calcium carbonate sediment dissolves at precisely the same rate as it is supplied from above, calcium carbonate dissolves at a faster rate than supplied below it, varies with water temperature

micrite

microcrystalline calcite matrix; fine-grained (finer that 4 microns in diameter) carbonate mud, analogous to matrix in wacke sandstone; polygenetic and is commonly formed by the physical disintegration of calcareous green algal masses within biologically produced needles of aragonitic are disseminated. also forms when allochems such as peloids and limeclasts disaggregate; it also can be the byproduct of mechanical abrasion and bioerosion (organic burrowing and ingestion) or carbonate grains. direct chemical precipitation

spar

microcrystalline sparry cement; relatively clear interlocking crystals of calcium carbonate, analogous to cement in arenite sandstone; forms as simple, primary, pore-filling cement or is generated secondarily by recrystallizating micrite

peloids

most are fine-grained (0.1 to 0.5 mm) sand- to silt-sized clasts of microcrystalline carbonate that lack a coherent internal structure; most are rounded to subrounded, but they can be spherical, subspherical, ellipsoidal, or irregular in shape; form in shallow marine low-energy platform carbonate settings such as the lagoonal areas of the Bahama Banks; originate in various ways; many are fecal pellets of waste matter generated by such organisms as fish and shrimp. Others are produced by the micritization of other kinds of allochems: ooids, oncoids, intraclasts (limeclasts), and abraded skeletal grains

oncoids

most are produced biogenically, modern examples have living encrustations of algae, foraminifers, and corals and form in various settings, including freshwater and brackish water reef complexes, shallow marine tidal flats and platforms, and (rarely) even deeper water carbonate settings; many large oncoids appear to be made of pieces of ripped-up algal mat that have been rolled around into an irregular ball

types of limestones identified using Dunham's

mudstone, wackestone, packstone, grainstone, and boundstone; all except boundstone accumulate as clastic carbonates-individual components are not bound together during deposition

brachipods, mollusks, foraminifers, and barnacles

only form their shells form stable calcium; well represented in the fossil record

organic activity

organisms extract their calcareous skeletons directly from seawater, plants photosynthesize (removing carbon dioxide)

light

photosynthetic organisms require light for photosynthesis, murky, muddy waters also inhibit them so they do not grow

limestones

probably the best documentation of organic evolution; dissolve easily, are often porous in the subsurface; deposition of it is happening at a much slower rate than it has in the geologic past

precipitation of limestone

promoted by any process that removes carbon dioxide from water; heated temperatures, reduced pressure (or depth) of seawater, breaking waves at the surf zone mix seawater with air (more CO2 in environment)

agitation by waves, tides, and storm currents

promotes the growth of ooids in shoal areas around the perimeter of the Bahama Platform and in the tidal channels and deltas of the Persian Gulf

dismicrite

recrystallized, bioturbated micrite

fossil components in limestone

reflect both age and depositional environment

sparry allochemical rocks

resemble arenites and consist chiefly of such allochems as skeletal fragments or ooids glued together by interstitial sparry cement

limestone clasts (or limeclasts)

ripped-up and transported fragments of pre-existing carbonate derived from extrabasinal and intrabasinal sources

Biolithes

rocks that were cemented together into limestone while the organisms that constitute them were still alive and growing; reef limestone and stromatolites; limestones that were crystallized directly from the activity of reef building corals or algae

Folk classifications

separates allochemical and orthochemical components

microcrystalline allochemical rocks

similar to wackes; consist of such allochems as skeletal fragments or ooids floating in or intermixed with fine-grained microcrystalline matrix (micrite)

The Folk Classification

sparry allochemical limestones and micritic allochemical limestones are subdivided on the basis of the kind and proportion of allochems and given composite names; more descriptive and objective

four principle limestone families

sparry allochemical rocks, microcrystalline allochemical rocks, micrite, biolithite

irregular thickness of individual laminae and their tendency to overlap

suggests that oncoids form under less uniform conditions than ooids

conditions where CO2 can be disolved

temperature, pressure (water depth), and degree of agitation, organic activity, sediment masking and clogging, light, and carbonate compensation depth

the sediment masking effect

the kind of sediment that accumulating at any point in time and space reflects what isn't happening as much as what is; more mud clogs gills of organisms that promote carbonate depostion, clay and mudstone being deposited rapidly (more than carbonate sediment), limestone can only form when other kinds of sediment are being deposited at very slow rates

oncoids (or oncolites)

the same size as ooids or may be larger, but they are irregularly shaped; they contain no obvious nucleus; individual coated laminae vary in thickness and show irregular development

neomorphism

the various diagenic processes of recrystallization and replacement, including changes in mineralogy-is very common in carbonate rocks

micrite under a microscope

translucent with a dull brown cast, dull and opaque in a hand specimen

biological community is determined by

water depth, water temperature, salinity, and turbidity