EARTH AND PLANETARY SCIENCES 101 , Fall, 2002

THE WAY THE EARTH WORKS

SEDIMENTARY ROCKS AND ASSOCIATED NATURAL RESOURCES

I. Why of importance?

A. hosts to hydrocarbon and other deposits

B. record of environmental conditions on the surface of the earth and their variations with time

C. record of life on our planet, and its evolution with time

D. usually the closest materials to our feet!!!

II. Definitions

A. Sediment

1. loose, solid particles

2. formed by weathering and erosion of pre-existing rock (as we know it), chemical precipitation, and secretion by organisms

3. different, in principle, from sedimentary detritus

4. Unconsolidated

B. Lithification

1. cemetation and compaction results in consolidation

C. Sedimentary rock

1. not just a rock formed from lithified sedimentary material

2. processes: lithification of any type of sediment, precipitation from solution, consolidation of remains of animals.

3. bears on the classification of sedimentary rocks: clastic/chemical/organic

III. Sediment

A. gravel-rubble-sand-silt-clay-mud (silt and clay): clay “grains” are taken as less than four microns in maximum dimension ( a micron is a thousandth of a mm, a clay-sized particle cannot be seen with the naked eye.

B. character of sedimentary material (weathering, erosoin, and transportation)

1. transportation----rounding, sorting

C. deposition--accumulation of detrital, chemical, or organic sedimentary rocks

C. environment of deposition---very important and reflects factors such as climate, plant and animal activity, etc.

IV. Types of sedimentary rocks

A. Clastic (typical term used, but let’s use the term DETRITAL instead)

1. detrital fragments, cementation, pore space, porosity, permeability

2. detrital texture

a. breccias vs. conglomerates

b. sandstone--medium grained, matrix partially to completely filled with cement

c. shale---fine-grained, usually fissile. If not fissile, called a mudstone. compaction is very important, pore space is reduced as grains are reoriented. CEMENTATION IS STILL THE MOST IMPORTANT MECHANISM FOR CONVERTING VERY FINE-GRAINED DETRITUS INTO A ROCK.

B. Chemical/Organic sedimentary rocks

1. chemical sedimentary rocks are precipitated directly from solution; inorganic/organic activity

2. organic sedimentary rocks contain remains of plants and animals

3. evaporites form by precipitation and crystal growth; crystalline texture and little primary sedimentary structure/texture

4. limestones

a. type of carbonate rock, consisting principally of calcium carbonate (calcite)

b. chemical vs. “clastic” origin---usually shallow marine incharacter

c. deposited in reef/lagoonal environments----shallow water, usually less than 60 m, at the very most.

d. recrystallization erases, so to speak, primary sedimentary textures.

e. note that with decreasing ocean water temperature (and therefore increasing water depth), calcite actually dissolves in ocean water, specific depth below which no calcium carbonate is “stable”

5. dolomite

a. magnesium partially replaces calcium in the carbonate structure.

b. generally a secondary material, formed by magnesium-rich fluids (brines) flushing through a carbonate system.

6. chert

a. silicon dioxide (quartz) is biologic in origin, usually

b. common as a sedimentary material on the ocean floor--remains of organisms called radiolaria.

7. coal

a. carbon-rich, remnants of plant material

V. Sedimentary Structures

A. Importance

1. orientation of a sedimentary sequence; which way is up?!!? What is the direction of “younging”?

2. primary or penecontemporaneous effects

3. bedding/original horizontality/bedding planes

a. reflecting changes in grain size and texture

b. reflecting changes in sediment composition.

4. Cross-bedding

a. deposition of detritus (usually sand-sized) on a relatively steep slope

b. wind dunes, sand bars in river/stream systems, etc

5. sediment grading.

a. particle size variations, turbidity currents (importance in a marine environment)

6. mud cracks

a. concave upwards--response to drying of fine-grained sediment at the surface

7. ripple marks

a. current (stream/river) vs. wave (beach)

8. fossils

a. preserved traces of plants (land plants after the Ordovician) and animals

II. Interpretation of Sedimentary Rocks

A. Source area/geography/ environment of deposition

B. source area

1. locality responsible for sedimentary material

2. controls are the types of rocks, climate, relief, rate of weathering

3. ancient vs. modern conditions

4. lots of possible ambiguities in interpretation, no question

C. Paleo (ancient) geography

1. location of source area, distance to the site of sediment deposition

2. study of the size and shape of rock units/sequences

3. example from the Salt Wash Member of the Jurassic Morrison Formation (home of the dinosaurs!)

a. source area was, in Jurassic time, the southwest part of the total area of distribution

b. highlands must have been to the west and south--probably an active mountain belt (compression related to tectonism in the western U.S.)

D. Environment of Deposition

1. lots of clues--size and distribution of individual deposits of sedimentary materials

2. channel vs. sheet type of distribution, as an example.

3. sequence of beds is often diagnostic

a. signify continental vs. marine deposition

b. coal/limestone/coarse sandstone (deltas probably flooded by oceans)

III. Sedimentary Facies

A. Lateral variations in rock types and/or fossil content within a single rock unit (i.e. sequence of sedimentary rocks deposited over a relatively short-lived period of time)

B. Salt Wash Member of the Lower Jurassic Morrison Formation

1. single “unit” deposited at the same time

2. three different facies and, by implication, relatively different depositional environment

3. conglomerate/sandstone/shale facies

C. In terms of carbonate rocks, limestones exhibit variability with respect to reef location, differ in source areas, etc.,

1. reef vs. land source in acarbonate environment

IV. Sedimentary Rocks and Earth Resources

A. Why are geologic resources important????

1. dependent on them

2. look at our backyard

a. NM state legislature decisions and state economy intimately tied to resources

3. Untapped resources? What are resources?

B. Concerns

1. rate of consumption and what has already been consumed, and is there a substitute?

C. Resources----geologically occurring and “valuable” materials

1. energy resources (curde oil, natural gas, coal, Uranium, geothermal, etc.)

a. great importance with respect to sedimentary rocks

b. of apparently greatest concern over the past two decades, continuing today!

2. metals (base and precious--Fe,Pb,Zn, Cu, Ag, Au, Pt, Mo, etc.)

3. non-metallic, non-energy (sand, gravel, clay, sulfur, gems, gypsum, etc.)

V. Energy Resources

A. constantly increasing consumption????

B. petroleum= liquid mixture of hydrocarbons

C. origin of oil and gas

1. organic matter is required (at least according to conventional wisdom)

a. rapid decomposition of sediment, so organic material is removed, essentially, from oxygen.

2. continental shelves and rise would be appropriate; lots of organic material

3. when oxygen is used up (or no longer available as a result of sufficient burial), then decay ceases and organic matter preserved in the sediment.

4. BURIAL/DIAGENENSIS/MATURATION/FLUID MIGRATION

D. Occurrences

1. conditions= source rock (rich in organic matter; anything greater than a few percent total organic carbon is a source rock according to oil people), reservoir rock, oil trap

2. recovery of crude oil---separates into petrochemicals of a range of types

3. expolration

4. environmental concerns (spillage, air pollution, etc.)

E. Oil Reserves (proven to exist); estimate will change every day

1. in 1985 (+/-), world situation was that there were reserves of some 500-600 billion barrels, with resources as about 2 trillion barrels. rate of consumption should allow the reserves to last some 20 to 30 years. but, I was told this in Fall, 1970. Concern that some resources are unrecoverable

a. what about today?---RESOURCES ESTIMATED AT 1.7 TRILLION BARRELS

2. in 1985, U.S. Consumption was some 6 billion barrels per year; with U.S. reserves estimated at 30 billion barrels and resources at 50 to 120 billion barrels.

a. what about today??? STILL, CONSUMPTION IS AT 6 BILLION BARRELS/YEAR WITH REMAINING US RESERVES AT ABOUT 26 BILLION BARRELS. RESOURCE ESTIMATES VARY CONSIDERABLY; THE MAX AVAILABLE IS ABOUT 100 BILLION BARRELS. MAX.

3. situation, again, in 1985

a. we imported 30% of what we consumed. TODAY WE IMPORT ABOUT HALF OF WHAT WE USE

b. natural gas: reserves at 200 to 400 trillion cubic feet, WITH RESOURCES ESTIMATED AT 7,400 TRILLION CUBIC FEET. we use about 20 tcf per year.

c. effect of price decontrol???? important for natural gas, as there are lots of different sources. Natural gas may exist at even deeper depths than oil; idea is that there are lots of conventional sources (not to mention a rather dubious hypothesis that most natural gas originates in the mantle)

4. Today

a. What is the corrent energy policy?

b. Should we open up more of the North Slope of Alaska?

c. Why is China making friends with Russia and the Persian Gulf nations?

F. Other possible sources of energy?

1. heavy crude oil---viscous residue of light petroleum

2. tar sands--asphalt-coated sands. Origin???? (U.S. 50 to 60 bb of recoverable tar sands)

3. oil shale---high total organic content locked in shale

a. Green River formation of Eocene age in Wyoming, Utah, and Colorado.

b. up to 650 m thick; estimated that it contains 400 billion barrels of oil--

c. lots of environmental problems with recovery

4. Coal--in 1985 was about 22 percent of our energy source (electricity); was higher.

a. future as a source of synthetic fuels????

b. origin--organic sedimentary rock--rapic plant growth, deposition in low oxygen environment; decay is inhibited. Sequence of development of high-C material: peat-lignite-subbituminour-bitumionuos-anthracite (anthracite is a metamorphic rock)

c. occurrence as a sedimentary rock

--eastern U.S. Appalachian basins--thin, anthracite

--interior U.S. undeformed, and un met, but deep

--western U.S. non-marine coals of late Cretaceous and early Tertiary age, low BTU and low sulfur, cheap to mine, but costly to ship

d. environmental problems--numerous (acid drainages, lower water table, pollution

e. reserves estimated at 400 to 800 billion tons; resources within 1.0 km of the earth’s surface at least 1.6 trillion tons.

f. consumption of about 0.8 bT per year.

5. Uranium

a. highly oxidized forms are soluble; reduced forms are very insoluble

b. minerals: yelow----carnotite, black --pitchblende

c. precipitation of uranium oxides and other compounds due to organic activity

d. areas of extreme interest in NM and Wyo; reserves of 500,000 tons of U oxide

e. plenty available to power 100 or so reactors in U.S.