Notes 2 Sedimentary Rocks

PgDip/MSc Energy Programme/Subsurface Sedimentary Rocks

The Robert Gordon University 2006 1 campus.rgu.ac.uk

Sedimentary Rocks

Review

In this topic we take a closer look at the sedimentary processand the resultant rock types.

Content

SedimentationSedimentation is the final process beginning with erosion andtransportation of eroded material to sites of deposition. Figure 1illustrates the process of sedimentation. Particles settle out of suspensionand are deposited horizontally as layers. Over time the layers areprogressively buried by overlying layers and the sediments are lithifiedinto rock.

Stratification results from the arrangement of sedimentary particles indistinct layers. Each stratum of rock is a distinct layer of accumulatedsediment. This arrangement of layers is known as bedding. Each stratumor bed has a bedding plane between it and the strata above and below.

Diagenesis is the name given to the chemical and physical changes thatoccur after deposition (Figure 2). These changes alter the texture andmineralogy of the sediment and act to convert the soft, unconsolidatedsediment into rock. Diagenetic processes include:

compaction, where water is driven out of the sediment (a mud with60% water content can become a mudstone with 10% watercontent);

recrystallisation of unstable minerals;

growth of clay minerals.



Figure 1. Erosion and Sedimentation. (From UNDERSTANDING EARTH byFrank Press and Raymond Siever, 1998, 1994 W.H. Freeman and Company. Used withpermission.)



Figure 2. Examples of Diagenetic Proceses. (From UNDERSTANDINGEARTH by Frank Press and Raymond Siever, 1998, 1994 W.H. Freeman and Company.Used with permission.)



Sedimentary rocks can be classified as Clastic (composed of fragments orparticles) or as Chemical (composed of minerals precipitated chemically).Breakdown of the parent rock by either mechanical or chemical means istermed weathering.

Clastic Sedimentary EnvironmentsClastic (detrital) sediments are formed from the breakdown of rocks byphysical weathering and transport of the resultant particles to lower lyingareas. The majority of clastic sediments are formed from the mechanicalbreakdown of pre-existing rock.

Mechanically eroded particles tend to be angular due to natural fracturepoints within the parent rock. However, as particles are transported theywill themselves be further eroded by abrasion. In addition, particles maybecome sorted by virtue of local conditions such as flow rate, particlehardness, size and specific gravity (Figure 3).

Figure 3. Sorting and Abrasion of Clastic Particles. (From THE DYNAMICEARTH by B.J. Skinner and S.C. Porter, copyright 2000 John Wiley and Sons. Thismaterial is used by permission of John Wiley and Sons, Inc.)

As time progresses, both roundness and sorting will increase. Many sandsfor example will consist almost entirely of well rounded, extremely wellsorted quartz particles, as quartz is one of the most resistant rocks toboth mechanical and chemical weathering. Bear in mind that thegeologist definition of well-sorted means that the grains are all of thesame or very similar size. Analysis of the layering and particles within a



sedimentary rock can give important information about the conditions ofsedimentation. For each environment we can list the distinctive physical,chemical and biological characteristics imparted to the resultant rock thatpermit us to distinguish it from rocks formed under a differentenvironment. Thus sedimentary rocks may be defined in terms of faciesin a similar fashion to that applied to metamorphic rocks (Figure 4).

Figure 4. Examples of Sedimentary Facies. (From THE DYNAMIC EARTH byB.J. Skinner and S.C. Porter, copyright 2000 John Wiley and Sons. This material is usedby permission of John Wiley and Sons, Inc.)

AlluvialThis environment includes river channels, meander belts on flood plains,alluvial fans and alluvial plains. As water flows it deposits material in slowmoving regions (inside channel bends for example) and erodes materialfrom faster flowing regions (outside bends). This leads to channelmigration over time. As a channel migrates it leaves behind a distinctive



sedimentary sequence, with coarse sand and gravel on the channel floor,grading into fine sands, silts and muds on the flood plain at the top(Figure 5). This is known as the fining upward alluvial cycle (Figure 6).

DesertThe desert environment is dry enough to allow sand to be blown by thewind. The wind blows sand into dune formations that migrate in thedirection of wind propagation. Dune sands are characteristically fine, wellsorted sands with cross bedding features (Figure 7). Dune deposits cangrade into alluvial deposits of desert rivers.

Figure 5. Deposits Formed due to Channel Meandering.

Figure 6. Upward Fining Alluvial Cycle Deposits. (From UNDERSTANDINGEARTH by Frank Press and Raymond Siever, 1998, 1994 W.H. Freeman and Company.Used with permission.)



Figure 7. Deposition of Cross Beds. (From UNDERSTANDING EARTH by FrankPress and Raymond Siever, 1998, 1994 W.H. Freeman and Company. Used withpermission.)

DeltaicDeltas are formed where rivers meet the sea and drop their sediment.The environment is characterised by the stratigraphic pattern of alluvialfreshwater deposits and fossiliferous marine deposits. Coarseningupwards of sediments may develop as the river mouth advances out intothe sea. Coarser deposits of the river will be progressively deposited overfiner offshore muds and silts (Figures 8 and 12). Large deltas may have acomplex structure of course stream channel sediments with fine sedimentlaid between and still finer sediment on the sea floor.

Figure 8. Typical Marine Delta.

Beach and BarBeach sands result from the motion of wave action and are generally wellrounded and well sorted. They exhibit bedding gently inclined towardsthe sea and oscillation ripples in the surf zone (Figure 9). A typicalsequence would be fine grained subtidal sediments overlain by medium-to coarse- grained tidal zone sand deposits, then beach sands and dunesands or salt-marsh organic-rich muds.



Figure 9. Oscillation Ripples. (From UNDERSTANDING EARTH by Frank Pressand Raymond Siever, 1998, 1994 W.H. Freeman and Company. Used with permission.)

Shallow MarineSedimentation on continental shelves is determined by wave bottoms andtidal currents: muds are deposited in depressions sheltered fromcurrents; sands and silts in areas of weaker currents; medium- to fine-sands in ribbons on shallower parts of the shelves. Large sediment driftscan occur along continental margins due to deep sea currents flowingnear the base of the continental shelf. Deep-sea fans to great depths canalso form if a river is aligned with a submarine canyon (Figure 10)although, in general, most land derived sediment deposition is confinedto the continental shelves.

TurbiditesTurbidite flows are formed by submarine slumps (Figure 11) as sedimentat the top of a slope is dislodged and moves as a turbid flow down theslope and out onto the abyssal plain. Turbidite sequences grade fromcoarse structureless sands to medium grained, bedded sands, then finersands and finally silts and muds (Figure 12). The fining upward sequenceis indicative of a waning current.



Figure 10. Deep Sea Fans. (From THE DYNAMIC EARTH by B.J. Skinner and S.C.Porter, copyright 2000 John Wiley and Sons. This material is used by permission ofJohn Wiley and Sons, Inc.)

Figure 11. Turbidite Currents.



Figure 12. Deltaic and Turbidite Cycles.

Clastic Sedimentary RocksClastics are composed of weathered particles or detritus, eg, shales,sandstone and conglomerates. Clastic deposits account for three-quartersof the Earths surface due to the dominance of mechanical erosionmechanisms over chemical precipitation. Shale is three times morecommon than any of the coarser clastics.

Fine Grained: MudsThese are the most abundant sediments on earth but due to their finegrain size they reveal least about their formation. The material is usuallystudied by electron microscopes and X-Ray diffraction. They are definedas sediments with a large component of clay-size material (



abundant organic matter, having formed in a poorly oxygenatedenvironment in which organic matter has not had the chance to decay.On burial this matter may alter to form oil and gas.

Medium Grained: SandstoneSandstones are classified on the basis of their grain size, texture andtheir mineralogy. If all the grains are the same size, the rock is said to bewell sorted. If the grains are of different sizes the rock is said to bepoorly sorted. Debris flow deposits are usually poorly sorted whereasbeach deposits are characteristically well sorted. Textural characteristicsalso include the roundness of the grain. This is a measure of how erodedor abraded the grain is. The more spherical and less angular the grain themore eroded and distal from its source. Table 1 shows sand classificationand associated grain size.

Table 1: Classification of Sand by Grain Size.

Classification Grain Size (mm)

Very coarse sand 1.0 2.0

Coarse sand 0.5 1.0

Medium sand 0.25 0.5

Fine sand 0.125 0.25

Very fine sand 0.0625 0.125

The mineralogy of the sandstone allows it to be traced back to its source.Quartz arenites contain almost entirely quartz grains. Arkoses containabundant feldspar. Lithic arenites contain lots of fine grained rockfragments from shales, slates, schists or volcanics. Graywackes consist ofquartz and feldspar grains surrounded by a finer clay matrix (Figure 13).



Figure 13. Sandstone Groups. (From UNDERSTANDING EARTH by Frank Pressand Raymond Siever, 1998, 1994 W.H. Freeman and Company. Used with permission.)

Course Grained: Gravel and ConglomerateThese consist of large pebbles and must have been deposited by strongcurrents with enough energy to pick up the grains in the first place, egmountain rivers. The limit that any river can carry is approximately 25cm in diameter. Pebbles are quickly abraded by transport and becomerounder and smaller the further they are transported. They may alsobecome aligned in the direction of current flow. Conglomerates (lithifiedgravel) therefore form in high energy environments, for example, duringstorms or on talus slopes. These are slopes at the foot of continentalmargins, coral reefs or mountains where boulders and debris accumulate.Because the clasts in gravels vary greatly in size, conglomerates areoften categorised by the size of the predominant grains, ie, boulder,cobble or pebble conglomerates. The clasts in conglomerates arecharacteristically rounded. However, if transport has been very brief, theclasts may be angular. In this case the resultant rocks are termedbreccias. Breccias also form due to the break-up of rocks along fault linesor of volcanic material during eruptions.



Chemical Sediments

CarbonatesThese are the most common chemical sediments, formed due to anabundance of calcium and bicarbonate ions in seawater. Limestone(CaCO3) is the most common carbonate rock. The related mineraldolomite (CaMg(CO3)2) is formed as magnesium rich waters percolatethrough limestone and diagenetically produce dolostone. Many marineorganisms, from one-celled animals to oysters, clams and otherinvertebrates, secrete some calcium carbonate. In this process ofbiological precipitation, the organisms extract calcium carbonate from theseawater to make their shells. Carbonate sedimentation is favoured inwarm, shallow tropical seas. Classification of carbonate rocks is not easyas they may be either clastic or chemical in character.

Coral ReefsReefs are thought to originate from corals and algae colonising the shoresof volcanic islands and to form a fringing reef. As the island slowly sinksdue to subsidence associated with sea floor spreading, the deposition ofcoral may keep pace with the sinking, gradually building up the reef.Eventually the volcanic centre disappears to be replaced by an atoll witha central lagoon (Figure 14).

Figure 14. Reef Growth.

Deep Sea OozesCalcareous or carbonate ooze forms over wide areas of the ocean floorwhere warm surface waters favour the growth of carbonate secretingorganisms. In deep water however (>4km) the level of carbon dioxidedissolved in sea-water leads to dissolution of any carbonate particles.Carbonate oozes lithify to chalk.

Siliceous ooze is formed near the ocean surface where siliceousorganisms predominate. This is often related to areas where deep,



nutrient rich, ocean waters rise and mix with surface waters. Siliceousoozes lithify to form chert.

EvaporitesThese are salts formed by the evaporation of seawater, such as halite(NaCl), gypsum (CaSO4 2H2O) and anhydrite (CaSO4). As seawaterevaporates, a sequence of salts is precipitated. The concentrated solutionformed at the surface from which evaporites precipitate is known as brine(Figure 15).

Figure 15. Evaporite Formation. (From UNDERSTANDING EARTH by FrankPress and Raymond Siever, 1998, 1994 W.H. Freeman and Company. Used withpermission.)



Tectonics and SedimentationSedimentation rates are high adjacent to regions of tectonic uplift,significantly lower on stable continental interiors and lower still in deepsea regions distant from terrestrial sediment sources. Exceptionally thickaccumulations of strata are related to specific plate tectonic scenarios. Inmost uplift areas, uplift rates exceed those of erosion and mountainranges are formed.

As continents diverge, sedimentary wedges may form along the newcontinental margins due to water run-off. Where continents collide,stream sediments from the resultant mountain ranges can accumulate inadjacent basins, and it turn, towards rivers and ultimately to the sea,forming deep sea fans for example. These sediments may become part ofthe continent and in turn be uplifted. Detritus from volcanic activity willalso add to this process (Figure 16).

Notes 2 Sedimentary Rocks

Documents

eroded particles

resultant particles

abrasion of clastic

parent rock

particles maybecome

sedimentary processand

robert gordon university

lithifiedinto rock