Sed Pet Week 14

carbonate diagenesis

Outsized pores

Large, irregular pores surrounded by smaller ones in a rock, often seen in carbonates. Might be preserved by calcite lining

Fenestral porosity

Happen in muddy carbonates, outsized pores are filled with cement. These pore spaces are kept open by gas bubbles escaping upward (from organic decay) that get trapped and allow for cementation 

Vadose zone

unsaturated (air filled) environment located between the surface soil and the saturated zone

Phreatic zone

Saturated zone below the soil. Often the source of aquifers

Carbonate ion concentration

If D > 1: the water is supersaturated (susceptible to carbonate formation

If D < 1: the water is undersaturated (not susceptible to carbonate formation)

If D = 1: water is exactly saturated

Aragonite dissolves faster than calcite. True?

True. This is why you see more carbonate ions in seawater in aragonitic environments; it is more soluble

in some cases, aragonite will revert to calcite

Carbonate dissolves more readily under

Cold, acidic conditions as opposed to warm ones (solubility increases with pressure and depth)

Normal marine salinity

3.5%; 35 grams of salt per liter

Fast vs. slow seafloor spreading rates (effect on Ca/Mg ratio)

Calcium to magnesium ratio in water increases (more Ca)

Calcium to magnesium ratio in water decreases (less Ca)

Shallow marine cements

Aragonite cements tend to form splays, acicular, botryoidal, and fibrous crustal structures (uncommon in ancient rock)

Magnesium-calcite cements tend to form stubby, bladed, and peloidal structures

Meniscus cement

Weak cements occurring in vadose environments (beachrock) that connect the points of grains. Often seen as carbonate lining that encrusts rocks you might pick up on the beach

Crinoid columnals tend to form

calcite overgrowths; they are calcitic and so their original structure is often preserved in thin section

Vadose meteoric cements

Meniscus cement, vadose silt, micritic envelope, epitaxial

Phreatic meteoric cements

Isopachous, blocky, epitaxial

Calcite cements occur prior to

Major compaction of pore space (shallow burial, early diagenesis)

Grapestones

Lightly cemented together in point to point contact in the depositional environment. These grains are then broken up by storm movements into circular chunks

Micritized skeletal grains

Type of peloid; skeletal grains form micritic growths in calm, shallow water environments

Carbonate lithics

Form as a result of sea level drops, exposing a reef that terrestrial rivers then erode. As a result, you get breakup of carbonate (limestone) into lithic chunks. Can also happen in the event of a wave-formed sea cliff

common in recycled orogen

Primary porosity is present during

Pre-depositional and depositional processes

Secondary porosity is present during (primary porosity absent)

Post-depositional processes

Eogenesis

Early burial; carbonates form at or before the deposition of the sediment itself (during deposition)

Mesogenesis

Deep burial; carbonates undergo mid and late diagenesis. Longest stage

Telogenesis

Carbonates are uplifted after diagenesis and exposed to the meteoric zone

Carbonate porosity types

Fabric or non-fabric selective

Moldic pore

Forms due to selective dissolution of a particular component (aragonite)

Stylolites

jagged, interlocking surface in a rock, most commonly found in limestone, that forms from pressure dissolution. Minerals that are insoluble, like clay or iron oxides, become concentrated along the surface while the more soluble minerals are dissolved, leaving a dark, visible seam that looks like a zig-zag suture