Background Information

• derived from bits and pieces of other rocks, or from chemical sediments
• can be found high on mountain ranges - result of uplift
• basically a surface event
• represent only 7% of Earth's crust, however, they cover 70% of Earth's surface
• in terms of volume of crust, are insignificant, but surface of continental crust is primarily covered with sedimentary rocks
• are not related to the movement of magma or tectonic activity; but are very much affected by tectonic processes
• form in horizontal layers, bedding planes (basic law of stratigraphy - #1)
• processes by which sediment is converted into sedimentary rock, including cementation and compaction = lithification
• are fossil carrying rocks, with most fossils found not on land but in shallow ocean rocks (due to greater likelihood of rapid burial)
• NOTE:  Origins differ greatly from igneous rocks (which are intrusive or extrusive)

• Rock debris - weathering; breaking of volcanic rock produces sediment
• Chemical sediments - water evaporates, salt left behind
(e.g.   TX & LA Gulf Coast have salt domes, very thick)
• Organic sediments - living corals are precipitating sediments; or swamp, plant material produces coal  (e.g.  large, black, smooth rock sample)

• Mud, sand and small pieces of decomposed rock (sediment) come from weathering
• gravity pulls them downhill
• Sand and pebbles settle down into nearly horizontal layers, accumulating on top of each other
• layer after layer is deposited
• Pressure combined with crystalline materials will eventually cement these deposits = sedimentary rock - calcium carbonate fills spaces between grains

Categories of Sedimentary Rocks

• clastic:  rock debris, bits and pieces of other rocks; mountain ranges eroding, river carrying bits downstream
• non-clastic:  chemical and/or organic sediments; shallow ocean floor

• conglomerate:    composed of rounded pebbles, origin related to running water; coarsest grain
• breccia:  broken/angular pebbles, origin related to mountain side avalanches; finer particles
• sandstone:  lithified sand, can tell us most about ancient environments (wind, temperature, etc.)
(e.g.  red coloration coming from hematite in class sample)
• siltstone:  finer sediments
• clay:  mud and clay solidify into sedimentary rock called shale; most common sedimentary rock

• limestone:  formed by the mineral calcite (major rock-forming mineral); major rock type not formed by silicate minerals; associated with tropical ocean, coral reefs: forms on continental shelves in warm, shallow tropical oceans
• is a major record for historical geology; during times in past the ocean has invaded deep into the continents...evidence for that is thick accumulations from limestone.  High on mountain ranges, we also find limestone; shallow ocean floor is caught into a converging plate boundary and deformed into mountain ranges
• (e.g.  Forms in Yucatan, Florida--no rivers flowing in; does not form along Gulf Coast where clastic sediments (silt and clay) literally kill coral reefs.)
• dolostone:  composed mostly of the mineral dolomite; can develop by direct precipitation from seawater, but most dolostones are formed by substitution of magnesium with calcium in limestone pour water
• rock salt and gypsum:  composed of mineral halite; forms by evaporation in saline lakes
• coal:  swampy environment begins to solidify, yielding peat (old decayed vegetation)...eventually it solidifies into a low grade of coal called lignite...then if pressure continuous it becomes bituminous coal (sedimentary rock--class sample)...and finally, anthracite (a metamorphic rock).

Features in the rocks that form at the same time that the rocks form  (See pps. 127-132)

• cross-bedding -  in wind-blown sandstone is produced by sand migrating over dune and accumulating on steep downward face.  Sandstone is exception to basic stratigraphy law...does not form in layers; characterized by cross-bedding.  (Figure F on p. 124)
• graded bedding -  produced by turbidity currents, in deep marine situation; underwater, downslope slide of sediment...the sediment sorts itself out by coarse to fine grain.  (Figure 6.10 on p. 131)
• ripple marks -  preserved in rocks like sandstone, indicate sediment deposited by current action of wind or water; provide information concerning the ancient beach environment.  (Figure D on p. 124)
• mud cracks - form where sediment dries while temporarily exposed to air; indicates hot, dry, evaporating conditions that cause ground surface to shrink; structure common on tidal flats
• e.g.  Tidal flats along Highway 48 leading to South Padre Island.   (Figure E on p. 124)

Sedimentary rocks can help identify what's happened in the past--marine environment is the one where you have best preservation.  (See pps. 136-141 for detailed photos of Environments of Formation)
 

• modern shallow-marine environment:  Bahama Islands
• ancient shallow-marine environment:  Eastern Kansas

• organic reef:  Moorea, west of Tahiti--extinct volcano surrounded by coral reef
• ancient reef deposit:  Australia, gradual subsidience of sea floor permits continuous upward growth

• modern deep-marine environment:  deep-sea fans off coast of India
• ancient deep-marine environment:  sediment on the southern coast of France

deltas - deposits of mud, silt, and sand that form at mouths of rivers, where they empty into the sea or lake;  very complex, horizontal beds of sedimentary rocks 

beaches - shoreline accumulations of sand; white sandstone made mainly from quartz; Veracruz & Pacific Coasts have dark sand because of volcanoes

barrier islands - linear bodies of sand built offshore by action of ocean waves; very temporary geological features  (e.g.  Padre Island)

lagoons - bodies of seawater located between mainland and barrier islands or reefs; a lot of mud, hence, not much mixing, results in shale and coal  (e.g.  Laguna Madre) 

tidal flats - shoreline areas covered with water at high tide and uncovered at low tide; mud is major type of sediment deposit; evaporating ocean water; evaporite deposits...halite and gypsum  (e.g.  Highway 48)
 
Continental Environments

alluvial fans - depositions of coarse sediments at the base of mountains  (e.g.  breccia)

eolian (wind) - sand dunes are built and transported by wind  (e.g.  Sahara Desert; Zion National Park, Utah)

fluvial (river) - relates to sediment carried by rivers; very important environment...sediments tend to be rounded   (e.g. resacas all over Brownsville are where the river used to flow -- they are called ox-bow lakes)

lakes - non-ocean water; including saline lakes in desert, and freshwater lakes on continental lowlands

glacial - areas where sediment is deposited by glaciers and their meltwaters; irregular, unsorted debris