5 - Clastic Rocks

the rock cycle

what are sediment?

• loose material produced on the earth’s surface by erosion and weathering, or as the result of organic or inorganic precipitation in diff enviros

• includes:

  • clays, silt, sand, gravel (siliciclastic)

  • shells (limestone)

  • evaporite minerals (evaporites)

how are sedimentary rocks formed?

• by compacting and cementation of sediment

• made up of small particles, stuck tgt w/ cement, w/ some pore space in bet

  • pore space usually filled w/ gas or fluid

what is grain size?

• card can be used to determine of sandstone is v fine, fine, medium, corase or v coarse

what is sorting ?

• grain size distribution

• sorting (well to poor)

• can tell us deg of transport n reworking

• poor sorting → close to source (proximal) little rework

• well-sorted → farther from source (distal) more transport and reworking

what is roundness ?

• measures smoothness of corners and edges

• better rounding indicates more transport and reworking farther from original source (distal)

beach gravel sorting and roundness?

• well rounded and well sorted

what is textural maturity?

• increase in rounding, sorting, and increase in stable minerals (e.g., quartz vs. feldspar and clay) is used to indicate a more “mature” enviro

• supermature quartz arenites commonly reflect multi-cycle sediments

  • i.e., deposited and re-eroded

what are the sedimentary rock layers referred to as?

• strata or beds

facts abt sediments and sedimentary rocks

• cover abt 70& of earth surface

• comprise only abt 5% of the vol of earth crust

• contaim most of the world’s energy resources (oil, gas, and coal)

• host most of the worlds subsurface aquifer’s

• contain fossils tat document the history of the dev of life on earth

what are the major calsses of sedimentary rocks?

• siliciclastic (terrigenous clastic)

• chemical/biochem and carbonaceous

what are siliciclastic sedimentary rocks?

• extraformational (allogenic)

• involves weathering and breakdown of large rocks

• transport usually involved

• derived from the “rock forming” silica-rich minerals

  • quartz, feldspar, clay-mineral, and rock fragment

categories of siliciclastic rock?

• conglomerate (<1% of all sedimentary rocks)

• sandstone (20-25%)

• mudstones/mudrocks/shales (siltstones and claystones 50%)

what are chemical/biochem and carbonaceous sedimentary rocks?

• intraformational (autogenic)

• usually formed as precipitate

• may be organic in origin

• carbonates (20-25%)

  • limestones Cac03

  • dolomites → ((Ca(Mg)Co3)

• largely formed by shells of orgas

• some inorganic chemical precipitation from fluids

  • TUFA, evaporite dolomite and limestone

what are evaporite?

• formed by chem precipitation from fluids (standing body ofw water)

  • gypsum (anhydrite, CaSO4)

  • rock-salt; halite (NaCl -salt)

  • sylvite (potash, KCl)

what is cherts?

• siliceous oozes

  • radiolarian accumulations

  • sponges spicules

  • diatoms

• diagenetic nodules D

ironstones, phosphorites, fossil fuels?

• ironstones

  • precambrain iron oolites

    • hematite (Fe2O3)

    • chamosite, chlorite, glauconite (iron silicates)

• phosphorites

  • guano (bap poop), bone beds, nodules

  • mostly apatite

• fossil fuels

  • coal, oil and gas

what is sedimentology?

• classification and interpretation of the origin of sediments and sedimentary rocks

  • e.g., mechanics of sediment transport, origin of sedimentary struct

    • requires physics and understand of fluid flow and turbulence

  • study of cementation that transforms sediment to rock

    • may require geochem and interaction of sediments, rocks, n fluids

what are sedimentary structures?

• fluid moves sediment as bedforms

  • e.g., ripples and dunes

• flume work and study of modern flows (e.g., rivers) shows that bedforms scale to the flow properties (e.g., velocity, water depth) and sediment type

• the understanding of the physics of sediment transport can used to to interpret ancient processes (water depth, velocities of flow)

how can ripples form?

• by water moving in one direction (current ripple)

• or back and forth (wave ripple)

• migration of ripples causes cross lamination

what are dunes?

• larger bedforms, tens of cm to m in height and wavelength

• scale to the depth of flow, flow velocity, and grain size

• migration of dunes forms cross bedding

• term “dune” does NOT imply formation by wind

• dines can be subaerial or subaqueous

  • above vs. below water

what are eolian dunes?

• underwater (subaqueous) dunes form in the same way but by water vs. air

what is stratafication?

• the product of processes active td are preserved in ancient sedimentary rocks → uniformitarianism

what are trace fossils?

• are marks made by orgas as they crawl, burrow or rest in or on sediments

• the study of trace fossils with a focus on sediment orgas interactions is called ichnology

• a single orga may make several diff traces

• the same trace fossils might be made by diff orgas

when looking at rocks, what is interpreted?

• physical or chem process (i.e., hydrodynamics)

  • e.g., storm, flowing water in a river channel

• place (i.e. enviro)

  • e.g., desert (aeolian), floodplain, reef, cold or hot climate

sedimentary enviros

enviros of deposition

facies concepts

• the sum aspect, appearance and characteristics of a sedimentary rock which arfe used to distinguish it from adjacent units and from the origin and the enviro of deposition may be understood

• facies come from word face = appearance

• categories

  • lithofacies, biofacies, ichnofacies, metamorphic facies

  • facies, sub-facies, micro-facies

facies example

• description:

  • medium grained, buff-coloured, cross stratified sandstone overlying mudstones containing fossil plants

• interpretation

  • sandstone formed as dunes by unidirectional flow. plant material suggest sand deposited in a terrestrial river channel

facies example - crawling burrows - cretaceous austin chalk

• description

  • grey, bioturbated muddy limestone

• interpretation

  • quiet water deposition in a shallow marine sea

facies - skolithos

• description

  • skolithos borrowed, tan-coloured, cross bedded sandstone. cross bedding dips in 2 directions

• interpretation

  • desposition in a shallow marine sea characterized by strong tidal currents

facies - jurassic morrison formation Utah

• description

  • red mudstone, cross stratified sandstones, erosional channel cutbank

• interpretation

  • red mudstones indicate an arid floodplain

  • sandstone suggests a fluvial channel eroded into floodplain

what is Walter’s law?

• “ only those facies and facies areas can be superimposed primarily that van be observed beside each other at the present time”

• explains how the vertical stacking of facies relates to enviros that were laterally connected in the past

example of Walter’s law?

• as shoreline migrate they produce a succession of facies that record the shallowing upwards

• deposits coarsen upwards

• delta front sands overlie silty foresets and muddy bottomsets

vertical measured sections

• we use measured sections to characterize sedimentary deposits

• in the case of rivers, sandstones are erosionally based

• fining upwards facies successions are dominant

• this records deposition during waning flows as rivers migrate across floodplains leaving their deposits behind

Walther’s Law facies successions

• vertical association of facies characterized by a progressive change in one or more parameters

• e.g.,

  • fining-upward channel fill succession

  • coarsening-upward prograding shoreface succession