EASC 446 Clastic sedimentary rocks lecture 1 PDF

EASC 446 The Rock Cycle: Clastic Sedimentary Rocks THE ROCK CYCLE Relative proportions of rock groups Sedimentary rocks: larger proportion of land surface The greater proportion...

EASC 446 The Rock Cycle: Clastic Sedimentary Rocks THE ROCK CYCLE Relative proportions of rock groups Sedimentary rocks: larger proportion of land surface The greater proportion created by weathering, erosion & deposition These are known as clastic sediments  SEDIMENTARY ROCKS Clastic sediments from weathering & erosion of pre- existing rocks Weathering can be physical (e.g, freeze-thaw) or chemical Erosion & subsequent transportation by water, wind and ice before deposition. Places of deposition could be desert sand dunes, deep ocean floors, river channels, river flood plains, or lakes, etc. Biogenic and chemical sediments form as biochemical or inorganic chemical precipitates E.g. shelly fragments in limestones; evaporites  Sediments to sedimentary rocks Weathering structures seen in ancient terranes Lithification due to: 1. Compaction Transport shown by (squeezing out water) sedimentary structures 2. Diagenesis (precipitation of cements)  Mineralogy of sedimentary rocks Clastic sediments siliciclastics – quartz, feldspars, clay Agate-Flint minerals, rock fragments calciclastics – calcite, dolomite Biogenic sediments calcite, aragonite chalcedony silica (flint)- form as a precipitate from moving solutions. Occurs as subsidiary Evaporite (chemical) sediments halite, gypsum and anhydrite Dolomite, also as dolomitisation of calcite Clay minerals such as kaolinite, smectite, illite, chlorite, micas are main components of raw materials of clay and formed in presence of water. Clay particles were found to be predominantly sheet silicate (phyllosilicate) minerals, now grouped together as clay minerals. Their structure is based on flat hexagonal sheets similar to those of the mica group of minerals. The planar structure of clay minerals give rise to characteristic platy habit of many and to perfect cleavage, as seen for example in larger hand specimens of micas.  Products of weathering Primary Residual Dissolved Minerals Minerals Ions Feldspar Clay minerals K+, Ca+2, Na+ Aluminum hydroxide Fe-Mg minerals Hematite & Mg+2 Limonite Quartz Quartz Silica Primary Solids that Ions that are carried Minerals remain in soil away in water ----------Detrital sediments------------------ Chemical & biochemical sediments  Siliciclastic Rocks: Texture Descriptive Textural Classification Grain Size Uden-Wentworth grain size scale Phi = -log2 (grain diameter in mm) naturally occurring groups Gravel ~ rock fragments Sand ~ individual mineral grains (particulate residues) Mud ~ particulate residues +/- chemical weathering products Clay ~ chemical weathering 9 products (clay minerals, etc.)  Detrital sedimentary rocks- classified by grain size differences Conglomerate Breccia Sandstone Shale All these rocks have clastic textures – the rocks are composed of particles (fragments) that are cemented together  Crystalline Finer grains between cement major clasts Grains floating in matrix Grains touching Quartz clast  Conglomerates & Breccias Conglomerates - mainly rounded clasts – Ortho conglomerates - clast supported – Para conglomerates - matrix supported Breccias - mainly angular clasts Clast type defines rock type: – Monomict - clasts of mainly one composition – Polymict - clasts of various compositions – Diamict - clasts poorly sorted of various types Matrix (sediment between major clasts) – sand and mud grade sediments Cement binds clasts/matrix (Crystalline, diagenetic, typically calcite, silica  Diamict conglomerate Orthoconglomerate Monomict breccia SANDSTONES CLASTS have modal grain size in the sand grade ( 2mm – 0.063mm) – Coarse sand 2- 0.5mm, – medium 0.5-0.25m – fine 0.25 - 0.063mm Siliciclastic sands have clasts composed usually of quartz, feldspar, fine grained rock fragments or lithics Mud grade matrix may be present Crystalline diagenetic cements, e.g. quartz, calcite, clay minerals  Three Primary sorts of Grains Quartz Feldspars Lithic Fragments  Maturity Maturity is a function of sediment transport Textural maturity refers to: – The degree of roundness of the grains – The amount of sorting of the grain sizes Texturally mature sandstones have well-rounded and well-sorted grains, immature if not…. Mineralogical maturity refers to the percentage of quartz grains – Feldspars break down with transport – Quartz grains more resistant Mineralogically mature sandstones have mostly quartz grains Arkose is mineralogically immature Goldich Reaction Series The Bowen reaction series/Goldich reaction series present similar mineral sequence. Bowen emphasises on crystallization/forms /color etc of mineral trends with temp etc. Goldich emphasises more on weathering trends with temp Sandstone Maturity  Roundness Dry(airborne aeolian) grains Increasing Roundness=increasing maturity Well-rounded & heavily abraded Sorting Well sorted Poorly sorted Increasing sorting=increasing maturity  Textural maturity – Clue to Erosion, Transport, Depositional history – Independent of composition! Maturity increases with energy input (same source) higher downstream Relative to starting material! clean sandstone can have high maturity (if rounded) Comparisons from different sources uncertain different starting grain size and shape distributions not comparable  Compositional maturity Measure of proportion of resistant or stable minerals present in the sediment, to less resistant minerals Sandstone with high maturity has mostly quartz Strongly influenced by composition of source rock area Resistant Less resistant Quartz Feldspar Chert clasts Most other minerals Zircon Lithic clasts Conglomerates & Breccias Sandstones Mudstones  Mineralogy Increasing quartz=increasing maturity  Classifying sandstones: mineralogy Quartz >95% Quartz = Quartz arenite 25% Feldspar = arkosic composition Arkose >25% Feldspar = mineralogically immature Quartz Feldspar Lithics PETTIJOHN CLASSIFICATION Based on percentage of matrix Matrix poor (15%) – wackes Wackes or (greywackes) Tend to be dark in colour Are poorly sorted Sandstone Classification Systems –The Pettijohn (1975) System EaES 350-3 30 Examples in hand specimen Light Light coloured coloured Light coloured Dark coloured Dark coloured Cross-bedded Red sandstone Lithic sandstone sandstone Polished sandstone Quartz sandstone (arenite) Interpreting the effect of depth to/with porosity Fine grained siliciclastic rocks Modal grain size 50% siliclastic grains,