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SEDIMENTARY ROCKS SEDIMENT –unconsolidated (“loose” loose”) granular geologic material SEDIMENTARY ROCK —lithified granular geologic material Sedimentary…
SEDIMENTARY ROCKS SEDIMENT –unconsolidated (“loose” loose”) granular geologic material SEDIMENTARY ROCK —lithified granular geologic material Sedimentary Processes SEDIMENT z Comes from: z – Physical weathering which creates particles – Chemical weathering which creates solutions z Sediment is unconsolidated Weathered and eroded material transported from higher to lower places Sediment sorting by a river SEDIMENT z z Particles are transported and deposited; solutions are transported and precipitated Transportation – Grain size – Rounding – Sorting Sorting = process by which particles having a particular size or characteristic are selected or concentrated. Which processes result in poorlysorted sediment? 1 Sedimentary Processes z Common Depositional Environments ENVIRONMENTS of deposition – MarineMarine- most abundant – Terrestrial River Marine shoreline and nearnear-shore environments Braided stream Alluvial fan deposits (terrestrial) Sediment moves downstream via migrating bars and in suspension during flood events Badwater alluvial fan Marli Miller Anastamosing channels—caused by sediment overload 2 SEDIMENT z Clastic Particle size – Gravel (> 2 mm)- Pebbles, cobbles, boulders – Sand (2 mm to 1/16 mm) – Silt (1/16 mm to 1/256 mm) – Clay (< 1/256 mm) ã Deposition ClayClay-sized particle vs. clay mineral TYPES OF SEDIMENTARY ROCKS Clastic – fragments accumulate z Chemical – precipitates accumulate z Organic – biogenic, lumped w/ chem in book z Common Sedimentary Rocks (Clastic) z CONGLOMERATECONGLOMERATE- z Breccia z SANDSTONESANDSTONE- z SHALESHALE- – From GRAVELGRAVEL- coarse rounded fragments… fragments… – From GRAVELGRAVEL- coarse angular fragments… fragments… – From SANDSAND- (1/16 mm to 2 mm)… mm)… – From SILT or CLAY (mud) – Most common sedimentary rock Conglomerate Breccia 3 Sandstone Bondi Beach <=Ss thin section Note: can’t see grains—feels smooth to the touch; if it contains some silt particles, then call it either a silty shale or mudstone (if 50/50 silt/clay) Lithification z z Lithification (compaction and cementation) Compaction Cementation – Cement deposited by groundwater – Cement most commonly calcite (CaCO3) – Less commonly silica (SiO2) – Iron oxide (hematite) when red Lithification of Shale Shale outcrop Bedford Shale, Cuyahoga River valley, Ohio, Mississippian age 4 Chemical sedimentary rocks Bioclastic limestone Other Sedimentary Rocks Chalk z COAL – partial decay of plants – important fossil fuel A soft marine limestone formed from the tests of many tiny protozoans, mainly foraminifers (aka “forams”) SEDIMENTARY STRUCTURES z BEDDINGBEDDING– Bedding plane— plane—an interuption in deposition z z Principle of Original Horizontality Principle of Superposition Bedding in sandstone and shale, Utah 5 Still sediment—not yet lithified! Bedded chert from CA coast ranges Peat layers, lahar deposits, and tephra east of Enumclaw Coal beds in Puget Group sedimentary rocks near Ashford Channel fill Sharon Conglomerate, Cuyahoga River valley, Ohio ~315 ma 6 Lava over sediment: Tieton Andesite lava flow (1 Ma) from Goat Rocks volcano over fluvial gravel along US 12 west of Naches Tilted beds What does the principle of “original horizontality” tell you about the history of these sedimentary strata? SEDIMENTARY STRUCTURES These features are indicators of processes and environments z z How crossbeds form— form—migrating dunes! CROSSCROSS-BEDDING (\ (\\\\\\): – Deposited by sand dunes via wind (terrestrial) – Deposited by river dunes (ex: in deltas) GRADED BEDDING – Caused by turbidity currents or by pulsing flow (glacial outwash) z z z MUD CRACKS RIPPLE MARKS ≈ ≈ ≈ ≈ FOSSILS Crossbedded sandstone MSOffice1 Planar cross beds Sharon Conglomerate, lower Pennsylvanian Age (~315 Ma), Cuyahoga River valley, Ohio; braided stream environment Zion National Park. Festooned cross beds deposited by wind. 7 Slide 39 MSOffice1 , 2/13/2008 Crossbeds in dipping layers of Ohanapecosh formation, volcaniclastic rocks; SR 165 west of Mt. Rainier Turbidity currents Deposited by one turbidity current D:\digital_content\animations_library\0009.swf Miocene Hoh Fm. Beach 4, Olympic Coast Upside down graded beds! 8 Mud up to 3 ft thick sits in fields, around homes and on roads in and around Curtis — and it's starting to dry up. Occupants of this Ceres Hill Road home are burning their furniture and belongings destroyed by the flood. STEVE RINGMAN / SEATTLE TIMES Ancient mud cracks in shale Modern mud cracks Current ripples in wet sediment, Baja CA Ripples Ripple marks in sandstone, Capitol Reef National Park, Utah Pat Pringle, photo Ripple marks in tilted Puget Group sedimentary rocks of Eocene age near Wilkeson WA 9 Fossil z …the remains of an organism preserved in rock. May consist of original mineral matter, petrified organic material, or an impression left behind. Above: Fossil fish from Eocene Green River Shale in western Wyoming Trilobite - species Chancia, order Ptychopariida, Cambrian Addy Quartzite, WA; David Morgan photo; Fossil flower: Eocene, Republic WA Florissantia uilchenensis Brachiopod Inoceramus Ammonites (Cephalopods) Wiwaxia, Pam Gore, cyanocacteria /paleolab/bivalvia.htm Marrella: Burgess Shale, Univ. of Calgary z Photos courtesy of R, Weller, Cochise College Fossil metasequoia leaf from rocks several miles east of Packwood, Oligocene age Dinosaur footprints, USGS /ovrdino.htm Glen Kuban’s site 10 Misc. Sedimentary Features z z z z Sorting Roundness Orientation: random vs. preferred Color Subfossil Douglas fir buried by Round Pass lahar from Mount Rainier along the Nisqually River (2.6 ka) Sorting Preferred orientation—shows current direction Imbrication = strong current <= Current direction = right to left Color A function of size and composition Finer grained = truer colors Weathering & matrix cement also influence z z z Red = oxidation (ex: hematite, “red beds” beds”— most of these are subaerial) Green = near oxidation/reduction margin (ex: glauconite, glauconite, chlorite) Black = reducing conditions (ex: pyrite, MnO2, organic matter) Rock Units z FORMATIONFORMATION– A geologic unit extensive enough to show on a map – Distinctive from neighboring rock units – Named locally, ex: Ohanapecosh Formation z CONTACTS – Sedimentary contacts – bounding surfaces between two sedimentary units 11 Unconformities—missing time! z z z Angular unconformity – horizontal beds atop tilted beds… beds… Disconformity – horizontal beds atop other horizontal beds, but missing time in between… between… Nonconformity – sedimentary rocks atop igneous or metamorphic rocks Monument Creek, Grand Canyon Angular unconformity Bob Webb, photo, 1986 Granite Rapid, Grand Canyon Grain size, sorting, and angularity in Tenino Sandstone Bob Webb, photo, 1986 12 Troutdale Formation Wildcat Creek beds US 12 near Rimrock Lake Centralia Coal Mine— Skookumchuck Formation 13
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