Lec 8 - Carbonate Rocks

Carbonate rocks include

limestones and dolomite

how much percent do carbonate rocks make up

25% of all sedimentary rocks

What time period did carbonate rocks begin to appear

pre-cambrian onward

How are carbonates formed

formed from biological and biochemical processes

where do carbonates form

shallow water within the photic zone 0-60m. Related to primary productivity of phytoplankton

Turbidity for carbonate formation

low turbidity. Enhances light penetration, prevents clogging of filter feeders, abrasion of shells by quartz.

sedimentological control

• mud smothers filter feeders

• freshwater input alters seawater salinity

what are the requirements for carbonates to form

Warm water >20^oC with consistent temperature, normal orceanic salinity

What areas do carbonates form in, Tropics and subtropics

within 30^o north and south of the equator

types of reefs

fringing reef, barrier reef, atoll

Controls on Carbonate deposition, describe the CCD

marks the water depth at which slowly falling CaCO3 sediment dissolves at precisely the same rate as it is supplies from the photic zone.

where do calcareous oozes accumulate?

Above the CCD, below it CaCO3 dissolves.

describe the classification of limestones based on grain size

Carcirudite >2mm

Calcarenite 2mm - 62 microns

Calcilutite <62 microns

How are limestones classified under Folk classification

prefixes indicate the framework grains present. Stems indicate whether the interstitial calcite is micrite or a sparite.

Biolithite is…

limestone formed in situ such as a stromatolite or reef-rock

dismicrite is…

micrite with cavities such as bird’s eye limestone

what terms did Embry and Klovan introduce to the Dunham naming system

bafflestone, bindstone, framestone, floatstone, rudstone

where are limemuds usually deposited

low energy environments: quiet lagoons, outer ramps, tidal flats, deep sea, basins and periplatform areas

how can lime muds accumulate in high-energy environments

if environment is trapped and stabilized by sea grass or a surficial microbial mat which leaves no record in the sediment

what are the two calcium carbonate polymorphs

Aragonite and calcite

characteristics of aragonite

orthorhombic, hardness of 4, metastable and alters to calcite over time. Most carbonate secreting organisms make aragonitic shells

characteristics of calcite

hexagonal (rhombohedral) crystal, hardness of 3, stable form of CaCO3, calcite may have been dominant skeletal material during the Paleozoic

other carbonate minerals

dolomite, siderite, magnesite, Rhodochrosite

components of sandstones

framework grains, matrix, cement, pore spaces

components of limestones

non-skeletal grains, skeletal grains, micrite, cement