Sedimentary Rocks

l l Products of mechanical and chemical weathering l l Account for about 5% of Earth’s crust l l Contain evidence of past environments s s Texture s s Fossils

Sedimentary Rocks Texture and composition - the keys to classification. Texture: Clastic or detrital Non-clastic Chemical (or crystalline) Biochemical

Sedimentary Rocks Clastic or detrital rocks are composed of particles of pre- existing rocks which have been weathered, eroded, transported, deposited, and cemented together. Grain size (energy) Rounding - energy and length of transport Sorting - uniformity of grain size

Sedimentary Rocks Chemical sedimentary rocks: precipitated from solution. Texture also referred to as crystalline- readily recognized in moderately coarse-grained examples. Very fine-grained chemical rocks appear massive. Some limestone, halite, sulfates, etc. Evaporites are a common source. Also caves, hot springs.

Sedimentary Rocks Biochemical rocks: formed by the processes of organisms Calcite or silica shells or skeletons. Form directly (coral reefs) or Cemented together after organisms die & accumulate Energetic wave action may break up shells and produce clastic textures = clastic or biochemical?? We'll consider it biochemical because the fragments are of biochemical origin If shell fragments are clearly recognizable, the texture is called skeletal

Sedimentary Rocks

Shale- fine grained (mud, silt, clay) Fissile (splits in layers) Shale with leaf fossils Sedimentary Rocks Clastic/Detrital Rocks Fissile Shale Low-energy environment: Offshore shallow marine Lake

Siltsone/mudstone- fine grained (mud, silt, clay) Massive- breaks in clumps Shale with leaf fossils Sedimentary Rocks Clastic/Detrital Rocks Fissile Shale Low-energy environment: Offshore shallow marine Lake

Sandstone- Sandstone- Composed of sand-sized particles Well-sorted: water or wind Forms in a variety of environments: beach, floodplain… Quartz is the predominant mineral Sedimentary Rocks Clastic/Detrital Rocks Thin-section of sandstone under the petrographic microscope

Sedimentary Rocks Clastic/Detrital Rocks Graywacke “Immature” sandstone Contains lots of easily-weathered material: Mafic minerals, volcanic glass/ash Typically occurs in subsiding basin near an active volcanic arc (uplift/erosion and burial > chemical weathering)

Sedimentary Rocks Clastic/Detrital Rocks Conglomerate- Pebbles/cobbles Well-rounded: long transport High-energy environments: Beach, River

Sedimentary Rocks Clastic/Detrital Rocks Breccia- Pebbles/cobbles Poorly-rounded: short transport High-energy environments: Beach, River, Fault zone Breccia

Sedimentary Rocks Chemical Rocks Some Limestone is precipitated directly from water Also true for some dolostone (Ca-Mg-carbonate) and chert (SiO 2 )

Sedimentary Rocks Chemical Rocks Some Limestone is precipitated directly from water Oolitic limestone contains small spherical ooids Travertine is typically precipitated from groundwater and is seen in caves

Sedimentary Rocks Chemical Rocks Evaporites are precipitated directly from seawater when a large quantity evaporates Rock salt Rock Gypsum

Sedimentary Rocks Biochemical Rocks Most Limestone is organic: precipitated as shells, reefs, and even as tiny shells from planktonic life Fossiliferous Limestones

Sedimentary Rocks Biochemical Rocks Most Limestone is organic: precipitated as shells, reefs, and even as tiny shells from planktonic life Chalk Cliffs Limestone Reef Chalk fossils under electron microscope

Sedimentary Rocks Biochemical Rocks Coquina is composed of broken-up shell fragments due to marine wave action in the near-tidal zone It also qualifies as a detrital rock

Sedimentary Rocks Biochemical Rocks Chert, like chalk, is composed of tiny shells from planktonic life, but the organisms involved secrete SiO 2 shells. Diatoms and radiolaria have microscopic SiO 2 shells

Sedimentary Rocks Biochemical Rocks Coal is organic: buried and un-oxidized plant remains

Sedimentary Rocks Biochemical Rocks Coal is organic: buried and un-oxidized plant remains

Sedimentary Rocks Sedimentary Environments Using sedimentary rocks to interpret Earth history

Sedimentary Rocks Sedimentary Structures Using sedimentary rocks to interpret Earth history Large-scale cross-beds: wind-blown sands in an arid environment Ripple-marks: running water- beach or river Many sedimentary structures may be used to determine original upward direction in vertical or overturned strata

Sedimentary Rocks Sedimentary Structures Using sedimentary rocks to interpret Earth history Graded beds- offshore turbidites

Sedimentary Rocks Sedimentary Structures Using sedimentary rocks to interpret Earth history Mud-cracks and raindrop imprints