Sedimentary Rocks — The Archives of Earth History ? Disconformity

There are actually 3 types of rocks… Igneous – cool from liquid (magma or lava) Metamorphic – pre-existing rocks that have been altered by intense temperature or pressure Sedimentary – form mainly from deposition of sediments

Historical geology focuses on sedimentary rocks •Why??? Only rocks that contain fossils Indicate ancient depositional environments

What is a sediment? Fragment of pre-existing rock (or animal shell) Why does water off Galveston look murky, while water off Florida looks clear?

What kind of rocks do we find around Houston? Not many rocks! Lots of unlithified sediment Why do many houses in Houston have foundation problems?

What is a sedimentary rock? Rock that forms at or near Earth’s surface 3 types Clastic Chemically-precipitated Biogenic

How do clastic sedimentary rocks form? Weathering Transport Deposition Lithification

How do other sedimentary rocks form? Chemical – precipitation of dissolved materials Biogenic (organic) – accumulations of organic material

Environments of Deposition At or near surface of Earth Marine Continental Transitional (deltas, barrier islands, beaches)

Marine Coastal Shelf Deep water www.ngdc.noaa.gov/mgg/image/sedthick.jpg

Continental Fluvial Desert Lacustrine Glacial Meandering Braided www.uoregon.edu/~millerm/depenv.html

Continental Fluvial Desert Lacustrine Glacial Meandering Braided www.uoregon.edu/~millerm/depenv.html

Continental Fluvial Desert Lacustrine Glacial Meandering Braided www.uoregon.edu/~millerm/depenv.html

Dune Cross-Beds Large-scale cross-beds in a Permian-aged wind-blown dune deposit in Arizona

Continental Fluvial Desert Lacustrine Glacial Meandering Braided www.mikelevin.com/DLBlissParkTahoe.jpg

Continental Fluvial Desert Lacustrine Glacial Meandering Braided www.peakware.com/encyclopedia/peaks/photos/everest7.htm

Moraines and Till Origin of glacial drift Moraines and poorly sorted till

Delta Form in oceans or lakes (marine and non-marine) www.uoregon.edu/~millerm/depenv.html

Stream/River-Dominated Deltas long distributary channels extending far seaward Mississippi River delta

Where would you find different sedimentary rocks?

Where would you find different sedimentary rocks?

Where would you find different sedimentary rocks? Grain size is controlled by energy High energy River Beach Low energy Lake Deep ocean Large grains Small grains

Sorting, Rounding If the size range is not very great, the sediment or rock is well sorted If they have a wide range of sizes, they are poorly sorted Wind has a limited ability to transport sediment so dune sand tends to be well sorted Glaciers can carry any sized particles because of their transport power, so glacier deposits are poorly sorted Grains more rounded with longer transport

Rounding and Sorting A deposit of well rounded and well sorted gravel Angular, poorly sorted gravel

Cross-Bedding Tabular cross-bedding forms by deposition on sand waves Tabular cross-bedding in the Upper Cretaceous Two Medicine Formation in Montana

Current Ripple Marks form in response to water or wind currents flowing in one direction asymmetric profiles allowing geologists to determine paleocurrent directions http://www3.interscience.wiley.com:8100/legacy/college/levin/0470000201/chap_tutorial/ch03/chapter03-5sedstr.html

Wave-Formed Ripple Marks As the waves wash back and forth, symmetrical ripples form Wave-formed ripple marks in shallow seawater

Modern Deposition near Houston Fluvial Brazos, Colorado, Trinity, San Jacinto Rivers Transitional Deltas, barrier islands Marine Gulf of Mexico

Ancient Environments Important for historical geology Important for oil companies (need to know where sand was deposited) Why are we looking at modern depositional environments?

Present is the key to the past Study modern depositional environments to learn about ancient ones Knowledge of ancient environments helps oil companies and historical geologists

Brazos River Longest river in Texas – 1450 km Highest sediment supply of any Texas river Originates in New Mexico

Where does deposition occur? www.uwsp.edu/geo/courses/geog391/toriv/Diagrams.htm

Point Bars Sediment deposited within the inside bank of a meander loop Fining upward sequence (grain size decreases) Coarsest sediment deposited by highest energy

Brazos River – Point Bar

Brazos River – Cut Bank

Coastal – Galveston Island

Barrier Islands Formed during sea level rise Rate of SL rise and rate of sediment deposition approximately equal Wave-dominated environment

Barrier Islands On broad continental margins with abundant sand, long barrier islands lie offshore separated from the mainland by a lagoon Barrier islands are common along the Gulf and Atlantic Coasts of the United States Subenvironments of a barrier island complex: beach sand grading offshore into finer deposits dune sands contain shell fragments (not found in desert dunes) fine-grained lagoon deposits

Barrier Island Complex Subenvironments of a barrier island complex

Texas Coast Most Texas beaches relatively fine-grained Low gradient of rivers like Brazos Why is this a problem?

Brazos Delta gulf.rice.edu

Environmental Interpretations and Historical Geology Present-day gravel deposits by a swiftly-flowing stream (Most transport and deposition takes place when the stream is higher) Nearby gravel deposit probably less than a few thousand years old

Environmental Interpretations and Historical Geology Conglomerate more than 1 billion years old shows similar features We infer that it too was deposited by a braided stream Why not deposition by glaciers or along a seashore? No evidence for either glacial activity or transitional environment