Sed-Pet Week 1

Porosity, composition, permeability

Porosity

Percentage of pore spaces in a mineral/rock

Permeability

Interconnectedness of pore spaces

Fabric

Arrangement grains and components in a rock

Phosphates

Rock made from phosphates like apatite

Carbonates

Rocks that are made of organic rich material. Form from precipitation of CACO3, susceptible to dissolution

Clasts

physical particles in rocks of varying sizes (4 microns - several meters) resulting from weathering and erosion

Siliciclastic sediment

Sedimentary rocks composed mainly of silicate minerals (SiO4) and derived from the weathering of pre-existing rocks. Not very susceptible to dissolution. Mainly made from gravel-sized grains

Broad classification of sedimentary rock

Siliciclastic rocks (silica-rich), chemical-biochemical rocks (formed from precipitation), carbonaceous (organic) rocks

Modern oil and gas exploration

Targeting shales - direct secretion of oil through breakdown of organic material. Previously, sandstone/limestone was the target

Textural maturity

How well rounded/sorted grains are, their shape. Mature might be rounded quartz grains, immature might be angular grains of various minerals

Compositional maturity

Composition of grains in a rock (mineralogy). Which grain types withstand a lot of weathering and which do not. Quartz used as indicator due to high stability at earth’s surface

Depositional environment and maturity

Low energy environments (floodplains, alluvial fan) tend to have less mature sediment due to lower ability to move grains around. High energy environments (beaches, sand dunes) can move grain much more easily and break them down, so you find more mature grains

Sediment basin

Accumulation of sediment at least 1 km thick and greater than 100k km in size

Extensional basin

Forms as result of extension from a normal fault. Plates slide past each other

Active margin

One plate moves closer to another and there is little space between them (on a fault line)

Passive margin

Plates move away from each other, but there is a lot of space in between it and its complementing plate (not directly on a fault line)

Diagenesis

The dissolution/destruction of some elements in sediment, introduction of new minerals, and eventual lithification

Humic materials

carbonaceous materials made of woody residues of plant tissue (chief component of most coals)

Sapropelic residues

remains of spores, phytoplankton, and macerated plant debris in water (chief constituents of cannel coals and oil shale)

Bitumens

Solid asphaltic residues that form from petroleum via loss of volatiles, oxidation, or polymerization

Sedimentary rocks began to form when

Earth’s atmosphere and oceans developed, since they helped in degassing the interior. Now they cover ~80% of land area

Most easily destroyed sedimentary rocks

Evaporates, due to their reactive nature and solubility. Next is limestone, then dolomite, shales, sandstones, and last, volcanic sediments

Constant mass model

Assumes early degassing of earth. All water in the hydrosphere and atmosphere were released at this time, along with other acidic gases that could react with igneous rock. Since that time, no totally new sediment has been created due to recyling

Linear accumulation model

Assumes water, CO2, and HCl are degassed constantly from the earth’s interior. New sedimentary rock has thus continued to form from igneous rock. Therefore, the mass of sediments has grown linearly from 0 to its current mass

Volcanic source rocks

originate in magmatic arc settings

Plutonic igneous rocks

originate in continental block provinces

Metamorphic/sedimentary rocks

Originate in orogenic belts from tectonic collisions

Accumulation space

space available in a sediment basin where sediment can accumulate at any time

Kinds of sedimentary basins

cont.

Tectonic setting governs…

The kinds of source rocks available to provide sediment to basins, and the composition of those sediments

Factors that control/affect depositional processes

Conglomerates

sedimentary rock composed of rounded pebbles, cobbles, or boulders larger than 2mm, cemented together by a finer-grained matrix of sand, silt, or clay

Ultra-stable clasts

Quartzite, chert, quartz veins

Metastable to unstable clasts

all other clasts besides quartz variants

Quartzose conglomerates

Conglomerates made from framework grains that consist of mainly ultra-stable clasts (90%)

Petromict conglomerates

Conglomerates that contain few ultra-stable clasts. More likely to be first cycle deposits

Most of the sedimentary rock record consists of

mudstone

Synthetic fault

Secondary fault associated with a larger, primary fault, where its direction of slip and dip are the same as the main fault. Occurs in extensional/strike-slip environments

Anti-synthetic fault

dips and moves in the opposite direction of the main fault. Occurs in extensional/strike-slip environments. Layers of sediment thicken toward the fault

Continental embankment

Feature of a continental margin; a ridge of sediment deposited at the edge of a continent where the shallow continental shelf gives way to the deeper ocean

Most faults do not progress into the

Post-rift sequence of an area. It is generally less deformed than syn and pre-rift strata

Rift to drift transition

geological shift from continental rifting, a process of crustal stretching and thinning, to seafloor spreading, where new oceanic crust forms at a mid-ocean ridge and the landmasses begin to drift apart

Progradation

to build out (more continental margin). Numbers indicate sediment age, with 10 being oldest

Salt diapirs happen because

Salt is less dense than the sediment surrounding it, so in cross section, you can see that it tends to move upwards in a dome like shape while folding the layers around it. Associated with rift and drift

Intracratonic rift basins

Depressions in old parts of earth's crust that form within stable continental blocks (cratons) due to extension. Show significant, long-lived subsidence and are filled with thick sequences of sedimentary rocks deposited by lakes and rivers. Located entirely in a landmass

Thermal contractive cooling

Rock cools as it moves away from the mid-ocean spreading center. As it cools, it shrinks and becomes more dense, causing it to sink lower and generate accommodation space

Inland rift settings

Denser volcanic (basaltic) sediments settle below in the trough of the rift while lighter sediments rise to the top

Aulacogens

Failed rift arms characterized by thick sections of shallow water sedimentation. The rift started to form, but could not form into an active oceanic basin (Mississippi river)

Oceanic trenches and slope basins

Formed by the flexure of the down-going slab in a subduction boundary. Dominated by accretion tectonic and oceanic sediments. May result in slope basins; subduction complex becomes highly deformed. Low geothermal gradient

Forearc

a region in a subduction zone between an oceanic trench and the associated volcanic arc

Remnant ocean basin

Dominated by deep water turbidites (flysch) and volcanics. Coarse tectonic clasts pile on top from erosion of continents; a small, shrinking basin that forms during the late stages of continental collision, where an oceanic basin is caught between two colliding continents or a continent and an island arc

Molasse

sedimentary sequence, typically found in foreland basins, characterized by clastic rocks like sandstones, conglomerates, and shales. Usually overlain on top of flysch

Foreland-style basin

Montana is mostly one of these. Sediment basin formed parallel to a mountain belt. Would see mostly sedimentary rocks here (sandstone, limestone, chert)

Restraining step

a step or bend in a strike-slip fault where the fault segments are oriented to produce local compression, causing crustal shortening, uplift, and mountain or hill formation (California)

Releasing step

discontinuity in a strike-slip fault system that creates a zone of extension. Causes the crust to subside and depress

How to recognize conglomerates

Form thick beds and often spires. Coarse grained and may be uniform in clast type or not. An example is the clasts in Yellowstone; many clasts are similar and appear to have been deposited by sediment flow

Fluvial/alluvial conglomerates

Clasts are very well rounded, and have many different colors. Clasts are also often touching (clast supported), and are imbricated. Not all are the same size but have similar composition

Glacial diamict

terrigenous sediment that is unsorted to poorly sorted and contains particles ranging in size from clay to boulders. Akin to glacial till; abundant mud matrix

Cata-clastic conglomerate

Formed as a result of tectonic activity (fault breccia, fold-thrust etc).