Non-Terrigenous Sediments and Rocks

1. Non-Terrigenous Sediments and Rocks Carbonate Sediments and Rocks

2. Most form as biogenic particles (essentially the only source) in warm (tropical; 30oN to 30oS latitude), shallow (shelf; within the photic zone), (mostly <10-20 m) marine water Also accumulate in deepwater ooze limestone (fine-grained) made up of skeletons of pelagic microorganisms such as Globigerina The Origin ofCarbonate Sediments

3. Why? Any process that decreases [H2CO3] forces rxn (2) to the left, CO2 is less soluble in warm water; CaCO3 has retrograde solubility H2O + CO2 <---> 2H2CO3 <–->H+ + HCO3- <-->H+ + CO3-2 CaCO3 + H2CO3 <-–> Ca+2 + 2HCO3-1(pH dependant reaction) The Origin ofCarbonate Sediments

4. Carbonates: General Characteristics • The majority of carbonate sediment forms in subtidal to supratidal environments and gives rise to widespread tabular deposits along continental (trailing edge) margins and epicontinental seas • Important occurrence in reefs, mounds or banks (bio-buildups)

5. bivalent metal cations Ca+2 Calcite, aragonite Mg+2 Magnesite, dolomite Fe +2 Siderite, ankerite + CO3 -2 The physical and chemical conditions of the environment in which calcites, dolomites and aragonites are formed are reflected in their composition. Carbonate Minerals in Sedimentary Rocks

6. bivalent metal cations Ca+2 Calcite, aragonite Mg+2 Magnesite, dolomite Fe +2 Siderite, ankerite + CO3 -2 Several factors control the crystal habits and crystal sizes of these minerals, most important: salinity ratio of magnesium to calcium of in the solution (time dependant) Carbonate Minerals in Sedimentary Rocks

7. Magnesian Calcite = small crystals > 4% Mg++ (4-9%), Mg++ = 3 x Ca++ in normal sea water (main site of CaCo3 ppt) “poisons” (interferes with calcite crystallization) and results in formation of meta-stable Aragonite (neomorphosis) Aragonite (orthorhombic polymorph of CaCo3) (Aragonite) (Mg,Ca)CO3 (Mg,Fe,Ca)CO3 Most modern Carbonate sediments (mostly biogenic)

8. Dolomite (CaMg{Co3}2) 42% - 58% CaCO3. Not readily formed at low temperatures; almost always 2nd ary mineral or very rare primary ppt “the dolomite problem”: Primary or Secondary/Authigenic?? Calcite: <4% Mg++ (CaCO3) (Aragonite) (Mg,Ca)CO3 (Mg,Fe,Ca)CO3 Most ancient carbonate rock is

9. Elevated temperature of formation favors dolomite alteration of limestone by most “normal” subsurface fluids The question really is: why aren’t all buried limestones dolomitized? Mass transfer processes play a decisive role in large scale dolomitization In lithified carbonate rocks effective fluid conduits for dolomitizing fluids (like fractures and primary facies) RULE! Fluid T-X Phase Diagram for Calcite & Dolomite Ca++/Mg++ ~ 0.3 in normal sea water From: Hardie, 1987

10. Dolostone Reservoirs • In carbonate systems, dolostone often forms the best reservoirs. • The dolomitization of carbonate sediments and rocks increases crystal size and pore throat size, and decreases pore roughness. • Without exception, the combination of these increases the permeability of the carbonate. • In this way, dolomitization “makes” the reservoir with dolomite serving as the reservoir and the surrounding limestone forming the seal. J.R. Allen and W.D. Wiggins, 1993

11. Hydrogeological Models for Dolomitization faults • The hydrology and mass transfer by “special” dolomitizing fluids • controls: • Size • Shape and • Location • of the dolostone body produced From: Alan and Wiggins, 1993; Modified from Land (1986) and Morse and Mackenzie (1990)

12. Other, (mainly diagenetic) Sedimentary Carbonate Minerals • Siderite • Fe Co3 • Ankerite: • Ca(Mg,Fe)Co3

13. Carbonates: General Characteristics • An antipathetic relationship exists between carbonate sediments and siliciclastic sediments due in large part to the biology of carbonate sediment-forming organisms • high terrigenous sedimentation rates increase turbidity, which inhibits photosynthesis by benthic organisms • gill breathers (such as the coral) get clogged up and die • The structures and textures of carbonate rocks mostly reflect intrabasinal, biological AND physical factors

14. Carbonates: General Characteristics • Carbonate sediments are particulate and subject to physical processes at the site of deposition just like TC sediments • Kinetic energy (currents) in the depositional environment influence: • grain size and sorting • BUT, carbonate sediments are mostly biogenic (“born” not “made”) and may not experience physical transport • This can confound grain size-sorting / depositional energy relationship

16. Source and Type of Sediments Produced in Modern and Ancient Carbonate Environments

17. The sand-sized grains that occur in carbonate rocks are called allochemical particles or allochems. Intraclasts Ooliths Peloids Skeletal particles (bioclasts) The interpretation of the depositional setting of carbonates is based on grain types, grain packing or fabric, sedimentary structures, and early diagenetic changes. Carbonate Rocks Constituents

18. Carbonate Rocks Constituents • The sand-sized grains that occur in carbonate rocks are called allochemical particles or allochems. • Intraclasts (early lithified carbonate fragments): • irregularly-shaped grains that form by syndepositional erosion of partially lithified sediment.

19. Carbonate Rocks Constituents • The sand-sized grains that occur in carbonate rocks are called allochemical particles or allochems. • Ooliths: concentrically laminated carbonate structures, including: • oolites -concentrically laminated structures,less than 2mm in diameter, thought to be abiogenic in origin • pisolites - same as oolites, but greater than 2mm in diameter • oncolites - spheroidal stromatolites (> 1-2 cm)

20. The sand-sized grains that occur in carbonate rocks are called allochemical particles or allochems. Pelloids: silt to fine grained sand sized carbonate particles with no distinctive internal structure; most thought to be fecal pellets Carbonate Rocks Constituents

21. The sand-sized grains that occur in carbonate rocks are called allochemical particles or allochems. Skeletal particles (bioclasts): whole microfossils, whole megafossils, broken shell fragments algae, forams, corals, bryozoans, brachiopods, gastropods, pelecypods, ostracods, etc. Standard microfacies (fossil fragment type -> environment) Carbonate Rocks Constituents

22. Carbonate Rocks Constituents • Micrite: • microcrystalline carbonate particles of clay (<1-4 micron) size (subtranslucent matrix) formed by • chemical or biochemical ppt • abrasion of allochems • implies deposition in a low energy environment just like in terrigenous mudstones

23. Carbonate Rocks Constituents • Cement: • sparry (twinkling crystalline) orthochemical material formed in interstitial pore spaces of “grainy” carbonate sediment • cement in pores indicates original void space • also recrystallized allochems or micrite

24. Carbonate Rocks Constituents • Insoluble Residues – • minor amounts of clay minerals and quartz occur in limestones, as insoluble residues, (so called because they do not dissolve in HCl) • Most insoluble material is chert (siliceous) • chert mostly originates from the shells of silica secreting organisms including diatoms, radiolarians, and some sponges.

25. Classification Schemes: Dunham Classification • Dunham Classification • Texture and allochem type incorporated into classification • sediment deposited in calm vs agitated waters • grain vs mud support • mud-bearing vs mud-free sediment • bound (biologically) • depositional texture recognizable

26. 5 Principles of Dunham Classification • Presence or absence of lime mud; is there any mud at all. Calm waters allow for the accumulation of lime mud and indicates the absence of current induced agitation • Grain Support: self supporting framework: • fluid circulation, diagenesis • Grain kind: standard microfacies types • Grain size, rounding, and coating: hydrologic interpretations • Biogenically ppt masses bound at time of deposition: • Boundstone • organic framework • laminations not consistent with gravity (stromatolite) • roof over sediment filled cavities

27. Dolomitic Rocks • Typically devoid of primary textures and structures; if primary textures are preserved • <10% dolomite: “dolomitized” (rock name) • >10% dolomite: dolomitic (rock name) • recrystallized carbonate: dolostone • saddle dolomite: “burial” dolomite of hydrothermal origin