Reading Question
1. Why is quartz more abundant than feldspar in sedimentary rocks?
Bioturbation
• Trace fossils - environmental interpretation
• Ichnofacies
• Paleobathymetry
• Must use caution - Occurrence, preservation, backfill
• Bioturbation index
Other Biogenic Structures
• Stromatolites
• Once the dominant form of reef
• Shape = environmental indicator?
Siliciclastic Rocks
• Silicate particles derived by weathering
• Extra-basinal
• Framework
• Matrix
• Cement
• Accessory Minerals
Framework Minerals
• Quartz, Feldspar, rock fragments (QFL)
• Mineral stability
• Quartz
• Mono versus polycrystalline
• Sources - plutonic, metamorphic, sedimentary recycling, volcanic
• Mature sediments
Framework Minerals
• Feldspar
• Angular; cleavage; optical properties
• Twinning
• Alkali feldspars
• Plagioclase feldspars
Framework
• Rock fragments (lithic)
• Volcanic
• Metamorphic
• Sedimentary
• Close proximity to course
• No recycling
Matrix
• Silt and clay minerals
• Illite, smectite, kaolinite
• Deposition
• Diagenesis
• Gluaconite - marine
Cement
• Diagenesis/lithification
• Quartz
– Syntaxial overgrowths
• Calcite
• Iron Oxide, clays, gypsum
Accessory Minerals
• Micas and “heavy minerals”
• Muscovite
• Heavies = mafics + zircon, rutile, tourmaline
• Inclusions
• Recycling
• provenance
Maturity
• Compositional maturity
• Textural maturity
• Degree of recycling (transport & reworking)
• Problem = diagenesis
Classification
• Siliciclastic classification
• Major groups = texture/grain size
• Gravel = conglomerate/breccia
• Sand = sandstone
• Silt = siltstone
• Clay = mudstone
Sandstones
• Sandstones
– R. Folk, R. Dott
• Framework versus matrix
– Arenites - wackes
• Percent of framework
– Quartz, feldspar, lithics
– QFL
Sandstones - Arenites
• Quartz arenite
– Multiple cycles of reworking
• Feldspathic arenite
– Arkose
– Close to source
• Lithic arenite
– Unstable fragments
– Tectonically active areas
Sandstones - wackes
• More than 5% matrix
• Close to source
• Most common = Greywacke (lithic)
– Tectonically active area
• Feldspathic wacke
– Active/unstable deep water basins
Conglomerate/Breccia
• Gravel (>2 mm)
• Conglomerates
– Oligomict
– Polymict
• Clast versus matrix supported
• Flat pebble cgl
• Breccia
– Epiclastic
– Vocanic
– Cataclastic
– solution
Siltstone/Mudstone
• Quartz, felspar, and clay minerals
• Siltstones
• Mudstones versus shale - fissility
• Make up large percentage of sed. Record
• Not well studied
• SEM
Siltstone/Mudstone
• Siltstones - gritty
• Mudstones versus shale - fissility
• Argillaceous, arenaceous, carbonaceous, calcareous
Provenance
• Mineralology - clues about source
• Rock fragments
• Feldspars
• Maffic minerals
• Metamorphic Minerals
• Quartz types
• Accessory Minerals and REE’s - inclusion studies
• Tectonic Setting
– Continental
– Recycled orogen
– Magmatic arcs
Diagenesis
• Post-depositional processes
• Burial
– Pressure
– Temperature (geothermal gradient)
– Pore water chemistry - salinty
Diagenesis
• Three stages of diagenesis (Choquette & Pray, 1970)
• Shallow Burial = Eodiagenesis
• Bioturbation
• Mineralization
– Pyrite, clays, glauconite, overgrowths - marine
– Iron oxides, kaolinite - nonmarine
Diagenesis
• Deep Burial - mesodiagenesis
• Compaction
– Loss of porosity - mechanical versus chemical
• Cementation
• Dissolution - secondary porosity
• Replacement - chert, calcite
• Dewatering
• Uplift - telogenesis