Where do Indiana s rocks come from? A short introduction to sedimentary geology

Transcription

1 Where do Indiana s rocks come from? A short introduction to sedimentary geology P. David Polly Department of Geological Sciences Indiana University Bloomington, Indiana USA pdpolly@indiana.edu Tidal rhythmites in Mississippian aged carbonates, Monroe County, Indiana

2 Objectives 1. Sedimentary rocks - definitions and sources 2. Siliciclastic versus carbonate 3. Depositional environments 4. Types of siliciclastic rocks (clays, siltstones/shale, sandstones, conglomerates) 5. Types of carbonate rocks (limestones, dolomites) 6. Rock types in Indiana 7. Denudation and erosion rates

3 The rock cycle Formation Weathering Transport Deposition Subduction Idaho, Joint School Districts 2 Nawrocki/Pages/Unit2RockCycle.aspx

4 Sedimentary rocks felsic rocks formed by weathering, transport, and redeposition of existing rock or by biotic precipitation Sedimentary rocks make up 75% of the surface of Earth, but only 5% of the crust. They occur only atop the outermost layers of the crust. Globally, sediments blanket continents to average depth of 3km and ocean basins to 300 m. Include unconsolidated weathering products (regolith or debris), soils (unconsolidated material that has been stratified by organisms and weathering) and lithified rock (cemented by chemical precipitates into rock). Sedimentary rocks have the greatest economic importance of the three primary rock types. Economically valuable sedimentary rocks include marble, crushed rock, paving stones, gravel, brick, lime, building stone. Sanders Quarry, Salem Limestone, Bloomington Indiana (2006, Wikimedia)

5 Major classes of sedimentary rock Siliciclastic rocks Composed of silica-rich minerals, mostly formed by weathering of igneous rocks, deposited by water and air flow. Typically deposited in rivers, lakes, deltas, or on continental coasts Siliciclastics weather into terrigenous debris that is eventually transported to sea. Carbonate Rocks Composed dominantly of calcium carbonate precipitated by the metabolic actions of organisms, deposited as organic buildup. Devonian aged siliciclastica, Jefferson County, Kentucky Carbonates typically precipitated in shallow marine waters. Carbonates weather by dissolving, after which streams carry the ions to the sea where they may reprecipitate. Mississppian aged carbonates, Monroe County, Indiana

6 Depositional environments places where weathered sediment accumulates or where carbonates precipitate Continental fluvial*, glacial*, desert Transitional deltas*, beaches, barrier islands Marine tidal flats*, carbonate banks*, reefs*, continental shelf*, continental slope, deep-ocean basin * Important environment for Indiana rocks Stanley, Earth System Processes

7 Siliciclastics Silici - with silica clastic - formed of clasts (fragments) Originate from material weathered from igneous rock, though particles may be recycled through several iterations of erosion and redeposition. Common clasts include clay, silt, sand, and pebbles. Quartz and clay minerals are common components. Classified by dominant particle size Stanley, Earth System Processes

8 Clast size depends on water flow The type of siliciclastic rock and its structures therefore provide evidence for the velocity and energy of past water flows Hjulström Diagram Critical velocites required to move grains on plane bed with water depth of 1 meter Boggs, Principles of Sedimentary Geology

9 Take-home message from Hjulström The larger the particle, the faster the water must be to move it. The larger the particle, the sooner it drops out of moving water. As water slows, it drops material in the following order: 1. Gravel 2. Sand 3. Silt 4. Clay 5. Dissolved calcium carbonate (reprecipitation depends on factors other than water flow) Stanley, Earth System Processes

10 Mississippi River Delta

11 Deltaic depositional environments Sediments include sands, silts, muds (sometimes carbonates) Stanley, Earth System Processes

12 Sorting of sediments by energy flow Coastal marine environments

13 Modern deltaic environments Mississippi River Delta, USA (the future destination of almost all Indiana rocks) Google Earth

14 Carbonate rocks Formed primarily of calcite or aragonite, precipitated from seawater or formed through the accumulation of skeletal remains of organisms (shells, corals, etc.) Carbonates are usually formed in marine, shallow water, warm environments Carbonate rocks include: Limestone, Chalk, Dolomite, Travertine, Oolites White cliffs of Dover, composed of chalk

15 Modern carbonate platforms and banks Bahama banks Google Earth

16 Evaporites Sedimentary rocks that precipitate from evaporating water Anhydrite and gypsum are the most abundant. Halite is another example (natural salt) Evaporation pools in San Francisco Bay Dolomite is a carbonate that may form in evaporitic conditions US Gypsum Mine, Shoals, Indiana St. Louis Fm., Mississippian Google Earth

17 Rock types in Indiana Sandstone - source was weathering of granite or other igneous rock, transport was by high-energy water or wind, probably represents ancient dune, river sand bar, or beach Siltstone source was weathering of igneous rock, transport was by mediumenergy water or wind, probably represents ancient river floodplain, swamp, or delta Shale source was weathering of igneous rock, transport was by low-energy water or wind, probably represents marine water below the wave base, farther from river mouth Limestone source was precipitation by organisms, no transport, probably represents shallow, tropical ocean floor Dolomite source was precipitation by organisms and alteration by very warm temperatures, probably represents shallow ocean floor in very warm, evaporating climate (or represents metamorphosis of ordinary limestone)

18 Dominant lithologies of Indiana s geological periods Ordovician - marine silts, sands and carbonates Silurian - carbonates and dolomites Devonian - carbonates and marine shales Mississippian - earlier: deltaic silts and sands, later: carbonates Pennsylvanian - cycles of silts, sands, coals, carbonates, silts, sands coals

19 Volume of sedimentary rock through the Phanerozoic Can be used to estimate the rate of loss due to erosion and subduction Wilkinson, B.H Humans as geological agents: a deep time perspective. Geology, 33:

20 Denudation rate (erosion) can be estimated from rate of loss Average = 24 m/my Wilkinson, B.H Humans as geological agents: a deep time perspective. Geology, 33:

21 Rates of sediment loss in historical times are two orders of magnitude higher Soil loss from cleared agricultural land, construction, etc. Average = 360 m/my Average = 24 m/my Wilkinson, B.H Humans as geological agents: a deep time perspective. Geology, 33:

22 Resources on soil loss in Indiana Soil erosion in Indiana, Purdue Extension Service No-till and strip tillage Indiana Association of Soil and Water Conservation Districts