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UMD GEOL 342 - Sandstones and Conglomerates

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Sandstones and ConglomeratesG342 Sedimentation and StratigraphyLecture 6: Sandstones and Conglomerates16 February 2006Assoc. Prof. A. Jay KaufmanSandstones and ConglomeratesBecause of their formation requirements (coarse detritus, strong current transport) andtheir physical properties (high porosity and permeability), the majority of sedimentary research hasfocused on sandstones and conglomerates. This is even though they represent, at most, 25% of thestratigraphic record. Below are sets of criteria and classifications that are handy in describing theselithologiesConglomerate and BrecciaLithified gravel and rubble are called conglomerate and breccia, respectively.Conglomerates are very special in the sedimentary record for two reasons. First, they are alwaysassociated with high energy systems. Second, they bring large chunks of material right to your feet.As such, they are very helpful in reconstructing provenance of the hinterland, and are often linkedto nearby tectonic activity.Orthoconglomerates consist primarily of framework grains and <15% matrix. Paraconglomerateshave a matrix of sand and finer clasts and are matrix-supported. The systematic orientation ofclasts in a conglomerate is termed imbrication. In conglomerate classification, there main concernis composition, but there are also textural issues around grain-size distribution and roughness ofgrains.1Diamictite is another term for a paraconglomerate, and is often used to denote glacialrocks. Clasts that appear to pierce finely laminated sediments were likely transported by sediment-laden icebergs and are called dropstones.In addition to a glacial origin for conglomerate and breccia, these coarse-grained depositscan also result from subaerial and subaqueous debris flows, subaqueous grain flows, turbiditycurrents, and bolide impacts.SandstoneSandstone is the indurated equivalent of unconsolidated sand containing clasts between 2 and 1/16 mm. Sandstones hold over ½ of the worlds oil reservesThere are MANY different classification schemes for sandstones, but the two definingparameters are the percentage of matrix and the composition of sand framework grains. Name Grain-size(mm)Φ Composition OtherQuartz arenite2 – 0.064 -1 to 4 >95% quartz Surprisingly commonArkose2 – 0.064 -1 to 4 >25% feldspar Often associated w/recycling of continentalcratonsLithic arenite2 – 0.064 -1 to 4 >25% lithicfragmentsOften associated w/recycling of arcs andmountain beltsGraywacke2 – 0.064* -1 to 4* 15-75% mud Often associated w/recycling of arcs and deepbasinsConglomerates>2 mm < -1 varied Round clastsBreccias>2 mm < -1 varied Angular clasts†Orthoconglomerate>2 mm < -1 varied Framework supportedParaconglomerate>2 mm* < -1* varied Matrix supported* actual mean and modal grain size varies with mud content† there are many different origins to breccias – be careful in interpretation.2Quartz areniteQuartz arenites are typically deposited in stable cratonic environmentssuch as eolian, beach and shelf. They are generally well lithified and wellcemented, and texturally mature to supermature. These sediments are mostlikely the product of multiple recycling of quartz grains (polycyclic). If theyare first generation it suggests an intense weathering environment.Feldspathic areniteA medium to coarse grained lithic arenite with >25% feldspar is knownas an arkose. These sandtonse form from the rapid disentigration of graniteto form grus, which is rapidly deposited, most often in alluvial fan deposits.They originate either 1) in cold or very arid environments where chemicalprocesses are inhibited, or 2) in warmer, humid environments where markedrelief or local uplift allows rapid erosion of feldspar before they can bedecomposed. A good nearby example is the Old Red Sandstone along bargecanal at Great Falls.GreywackeArenites are differentiated from wackes by the percent of matrix grains. Wackes contain>15% and arenites less. The proportion of rock fragments to feldspar in wackes is believe to be a3reliable index of sandstone provenance, one supracrustal (i.e. volcanics and fine-grainedsedimentary rock) and the other subcrustal (i.e. granite and gneiss)Textural maturity of siliciclastic rocksThere are four stages of textural maturity based on 1) the proportion of clay clasts, 2) thesorting of the sand grains, and 3) the roundness of the sand grains. Sandstone is texturallyimmature if there is >5% clay clasts, regardless of sorting or rounding. These also tend to be richin feldspar, and are common in alluvial fans, turbidites, and overbank deposits. Submaturesandstones have <5% clay but are otherwise similar to immature sandstones. Mature andsupermature sandstones also have <5% clay, but the sand framework is well rounded and sorted.These are common in fluvial channels, beaches, and aeolian dunes. Grain textures are the chief factors that control the porosity and permeability of sedimentsin siliciclastic settings. These include 1) grain size distribution (mean, median, and sorting), 2)shape (sphericity), 3) packing, 4) composition, and 5) cementation. Porosity is the % of voidspace,P = (bulk volume – grain volume)/bulk volumePorosity is NOT constrained by geometry. The distribution of pores in a volume isindependent of porosity. In general, better sorting results in better porosity, and finer grain sizesshow higher porosity.4Permeability is the “flowability” of fluids within a volume of rock and is expressed inDarcy units (cm/s). Permeability is STRONGLY constrained by geometry. The governingparameter is pore-throat diameter, or the size of the minimum space between grains. As such,permeability is affected both by sorting and mean grain size. A heterogeneous fabric diminishespermeability by increasing the tortuosity of the pore system. Permeability is not flow rate, whichvaries as a function of viscosity, temperature, and pressure.Packing is the mutual spatial relationships between grains. This includes packingproximity, or how many grains another grain touches, and packing density, or the cumulativenumber of grains intercepted by a random line through the volume. Rocks with identical grain-sizedistributions can have different porosity and permeability as a function of packing.


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