Sedimentary Rocks, Earth's Thermostat, and Marine Carbonates

Comprehensive vocabulary flashcards covering clastic rocks, sandstone classification, carbonate microfacies, evaporites, coalification, Earth's silicate weathering thermostat, Cryogenian Snowball Earth, and marine Calcite/Aragonite seas.

Grain-size classification of clastic rocks

The textural categorization of clastic sedimentary rocks based on diameter into conglomerates/breccias ($>2\,\text{mm}$), sandstones ($0.064\,\text{to}\,2\,\text{mm}$), and mudrocks ($<0.064\,\text{mm}$).

Mudrock grain size boundary

Grain size threshold of less than $0.064\,\text{mm}$, defining rocks composed primarily of mineral grains and clays.

Sandstone grain size range

Clastic grain size spanning from $0.064\,\text{mm}$ to $2\,\text{mm}$, consisting predominantly of mineral grains and rock fragments.

Conglomerate and breccia grain size boundary

Coarse sediment grain size exceeding $2\,\text{mm}$, consisting predominantly of rock fragments.

Mudrock constituents

The primary compositional ingredients of mudrocks, consisting of fine-grained mineral grains and clay minerals.

Sandstone constituents

The framework components of sandstone, predominantly individual mineral grains (crystals) and rock fragments.

Conglomerate and breccia constituents

The coarse framework grains of conglomerates and breccias, which are predominantly rock fragments.

Conglomerate and breccia texture and composition schemes

The classification of coarse clastic rocks based on clast shape (rounded vs. angular), packing texture (clast-supported vs. matrix-supported), and clast lithology (monomict vs. polymict).

Conglomerate (clast shape)

A coarse clastic sedimentary rock with grains larger than $2\,\text{mm}$ that are rounded and smooth.

Breccia (clast shape)

A coarse clastic sedimentary rock with grains larger than $2\,\text{mm}$ that are sharp and angular.

Clast-supported texture

A sedimentary fabric where larger clasts touch one another because there is very little fine-grained matrix between them.

Matrix-supported texture

A sedimentary fabric where larger clasts are separated from each other and float within a groundmass of much smaller grains.

Beach shingle in high-energy surf

A modern depositional setting where high wave energy abrades and rounds large rock clasts into smooth pebbles over time.

Monomict clast composition

A coarse clastic rock where the gravel-sized clasts are derived entirely from a single rock type.

Polymict clast composition

A coarse clastic rock containing clasts representing a variety of different rock types.

Transport interpretation of conglomerate

Indicates a longer duration of high-energy transport that allowed abrasion to round the clasts.

Transport interpretation of breccia

Indicates short high-energy transport with little abrasion, representing rapid erosion and deposition near the source.

Alluvial fan conglomerate specimen

A conglomerate recording an arid alluvial fan with flowing water, high energy, and brief transport that rounded abradable rock fragments.

Mudrock softness rule of thumb

A general geological principle stating that the finer the grain size of a sedimentary rock, the softer it tends to be.

Mudrock cliff-forming potential

Mudrocks tend not to form large cliffs because they are relatively soft, though cliff formation is possible in rare cases.

Claystone grain size

Extremely fine-grained mudrock where individual grains are less than $0.004\,\text{mm}$ in size.

Claystone tactile texture

Characterized by a smooth feel when rubbed between fingers or tested against teeth.

Siltstone grain size

Relatively coarse mudrock with grain sizes ranging from $0.004\,\text{mm}$ to $0.062\,\text{mm}$.

Siltstone tactile texture

Characterized by a distinctly gritty feel when rubbed between fingers or teeth.

Mudstone versus shale comparison

The textural distinction in mudrocks between massive, homogeneous blocks (mudstone) and thinly layered, fissile sheets (shale).

Mudstone (textural definition)

A mudrock characterized by a massive, homogeneous texture that breaks into blocky lumps rather than thin sheets.

Shale (textural definition)

A mudrock characterized by fine laminated sheets that exhibits fissility, splitting easily along closely spaced planes.

Fissility

The property of a rock to split easily along thin, closely spaced, parallel bedding planes, typical of shale.

Mudrock depositional environments

Low-energy hydrodynamic settings where fine particles slowly settle out from water, including lakes, river floodplains, tidal mudflats, and deep ocean basins.

Grades of sand comparator

A calibrated visual scale illustrating sand sizes from very fine to very coarse ($0.125\,\text{mm}$ to $2\,\text{mm}$) as well as gravel grades.

Logging card true scale

The printed symbols on a geological logging card representing sand grain sizes accurately at a 1:1 true scale.

Very fine sand grade

Sand grains with a diameter of approximately $0.125\,\text{mm}$.

Fine sand grade

Sand grains with a diameter of approximately $0.25\,\text{mm}$.

Medium sand grade

Sand grains with a diameter of approximately $0.5\,\text{mm}$.

Coarse sand grade

Sand grains with diameters ranging from approximately $0.75\,\text{mm}$ to $1.0\,\text{mm}$.

Very coarse sand grade

Sand grains with diameters ranging from $1.5\,\text{mm}$ to $2\,\text{mm}$.

Granule size grade

Clastic sediment grains ranging from $2\,\text{mm}$ to $4\,\text{mm}$ in diameter.

Pebble size grade

Clastic sediment grains ranging from $4\,\text{mm}$ to $64\,\text{mm}$ in diameter.

Cobble size grade

Clastic sediment grains ranging from $64\,\text{mm}$ to $256\,\text{mm}$ in diameter.

Boulder size grade

Clastic sediment grains exceeding $256\,\text{mm}$ in diameter.

Plagioclase polysynthetic twinning under cross-polarised light

Diagnostic striped 'barcode' extinction texture visible in plagioclase feldspar grains between crossed polarizers.

Diagnostic feldspar texture

A distinctive 'barcode' twinning texture observed under cross-polarized light (XPL), particularly diagnostic of plagioclase feldspars.

Optical properties of quartz

Described petrographically as 'boring' because it lacks internal texture, cleavage, and pleochroic color in both PPL and XPL.

Lithic sandstone in thin section

A sandstone thin section dominated by heterogeneous rock fragments, clearly discernible under plane-polarized light.

Quartz arenite

A sandstone composed almost entirely of quartz framework grains with minimal matrix and negligible feldspar or lithic clasts.

Sandstone matrix

Fine-grained clay material occupying the pore spaces between larger sand framework grains.

Sandstone cement

Mineral matter precipitated post-depositionally from groundwater (commonly quartz or calcite) binding sand grains together.

First-order sandstone classification

The primary division of sandstones based on the proportion of clay matrix: arenites have $<15\%$ clay, whereas wackes have $15-75\%$ clay.

Arenite

A sandstone containing less than $15\%$ fine-grained clay matrix, meaning framework grains are clast-supported and touching.

Wacke

A sandstone containing $15\%$ to $75\%$ fine-grained clay matrix, where quartz and feldspar grains float in groundmass.

Second-order sandstone classification

The secondary division of arenites and wackes based on the relative proportions of Quartz (Q), Feldspar (F), and Rock fragments (RF).

Arkosic arenite

An arenite ($<15\%$ clay matrix) whose framework grains are enriched in feldspars over rock fragments.

Lithic arenite

An arenite ($<15\%$ clay matrix) whose framework grains are dominated by rock fragments over feldspars.

Arkosic wacke

A matrix-rich sandstone ($15-75\%$ clay) where the framework grains contain abundant feldspars.

Lithic wacke

A matrix-rich sandstone ($15-75\%$ clay) where the framework grains contain abundant rock fragments.

Framework grain physical and chemical durability comparison

A comparative summary of Mohs hardness, cleavage planes, and chemical resistance among quartz, feldspar, and rock fragments.

Mohs hardness of quartz

A hardness value of $7$ on the Mohs scale.

Cleavage planes of quartz

Zero cleavage planes; quartz breaks along irregular or conchoidal fractures.

Chemical resistance of quartz

High chemical resistance, making quartz extremely durable against weathering during surface transport.

Mohs hardness of feldspar

A hardness value of $6$ on the Mohs scale.

Cleavage planes of feldspar

Possesses $2$ distinct planes of cleavage, facilitating mechanical breakdown along weakness planes.

Chemical resistance of feldspar

Low chemical resistance; feldspar readily alters to clay minerals such as kaolinite when exposed to acidic water.

Hardness of rock fragments

Not strictly applicable as a single value due to polymineralic makeup, but generally low overall durability.

Chemical resistance of rock fragments

Low chemical resistance because polymineralic and fine-grained fabrics weather quickly.

Sedimentary grain roundness and sphericity chart

A comparative reference diagram showing grain roundness stages from very angular to well rounded across high and low sphericity.

Roundness (definition)

The degree to which sharp corners and edges of sedimentary grains have been worn away (abraded) by collisions during transport.

Relationship between roundness and transport

Roundness increases with the duration of transport and the hydrodynamic energy of the depositional environment.

Sedimentary texture components

The combination of grain size, grain shape (roundness), and the range of grain sizes (sorting).

Sorting

The degree of uniformity in the distribution of grain sizes within a sedimentary deposit or rock.

Poorly sorted sediment

Sediment containing a wide range of different grain sizes mixed together.

Well sorted sediment

Sediment possessing a narrow, uniform range of grain sizes.

Relationship between sorting and transport

Sorting increases with the duration of transport as currents have more time to segregate particles by size and density.

Textural and mineralogical evolution during sediment transport

A schematic showing that downstream transport progressively rounds grains, sorts grain sizes, removes clay matrix, and enriches quartz.

Mineralogical maturity

The extent to which a clastic sediment's composition is dominated by chemically stable quartz over unstable feldspars and lithics.

Mineralogically mature sandstone

A sandstone enriched predominantly in durable quartz, indicating extensive weathering and prolonged transport.

Mineralogically immature sandstone

A sandstone containing abundant unstable feldspars and rock fragments, reflecting brief transport or rapid erosion.

Four major classes of sedimentary rocks

The classification of sedimentary rocks into clastic, organic, biochemical, and chemical types based on constituent origin.

Clastic sedimentary rock (broad definition)

Sedimentary rock formed from accumulations of solid debris and fragments derived from pre-existing weathered rocks.

Organic sedimentary rock (broad definition)

Sedimentary rock composed of the debris and residues of organic material, such as plant fragments (e.g., coal).

Biochemical sedimentary rock (broad definition)

Sedimentary rock formed from accumulations of mineral material generated by biological metabolic activity (e.g., limestone).

Chemical sedimentary rock (broad definition)

Sedimentary rock composed of accumulations of mineral crystals precipitated directly from aqueous solution (e.g., halite).

Autochthonous nature of non-clastic rocks

Biochemical, chemical, and organic sedimentary rocks are usually deposited directly where their constituents formed (e.g., lagoon, peat bog, salt flat).

Relevance of sorting and roundness in non-clastics

Because there is minimal physical transport, textural parameters like roundness and sorting have little diagnostic relevance in non-clastic rocks.

Mineralogical simplicity of non-clastics

Non-clastic sedimentary rocks are mineralogically simple, often composed entirely of a single mineral species.

Mineralogy of young limestones

Young or modern limestones frequently contain aragonite along with calcite.

Mineralogy of ancient limestones

Ancient carbonate rocks are composed predominantly of calcite and/or dolomite.

Chemical formula of aragonite and calcite

$\text{CaCO}_3$ (calcium carbonate).

Chemical formula of dolomite

$\text{CaMg}(\text{CO}_3)_2$ (calcium magnesium carbonate).

Clastic rock genetic sequence

Weathering & erosion (usually on land) $\rightarrow$ transportation (rivers, ice, wind, oceans) $\rightarrow$ deposition $\rightarrow$ lithification (long after deposition).

Carbonate rock genetic sequence

Mineral precipitation (usually in the ocean) $\rightarrow$ brief transportation $\rightarrow$ deposition $\rightarrow$ lithification (upon or shortly after deposition).

Lithification timing comparison (clastic vs carbonate)

Clastic rocks lithify long after burial, whereas carbonate rocks typically lithify upon or very shortly after deposition.

Limestone versus sandstone petrographic comparison

A comparative overview showing that limestones consist of allochems, micrite, and calcite cement, whereas sandstones consist of silicate grains, clay matrix, and mineral cement.

Allochem

A distinct, recognizable carbonate grain in a limestone, such as a bioclast, ooid, or coated grain.

Bioclast

An allochem consisting of an intact or fragmented fossilized shell or skeletal element of a marine organism.

Ooid

A millimeter-sized spherical carbonate grain formed by concentric layers of fine-grained carbonate mud (micrite).

Coated grain

An irregular or millimeter-sized carbonate grain coated by fine-grained carbonate mud (micrite).

Micrite

Very fine-grained aragonite or calcite carbonate mud occupying spaces between allochems.

Sparry calcite cement

A coarsely crystalline calcite cement precipitated in void spaces between allochems.

Limestone depositional and diagenetic cycle

A schematic of limestone formation depicting inorganic and organic precipitation, deposition, transport, and burial diagenesis.

Aragonite shell diagenetic void infilling

Unstable aragonitic shells dissolve away after burial, creating open molds that are subsequently infilled by crystalline calcite cement.