GEOLOGY ROCK EXAM: SEDIMENTARY

Explain how biochemical sedimentary rocks form.

A variety of organisms in marine environments produce mineralized skeletons for their protection. When these organisms die, their skeletons often become fractured into grains by the activities of waves, currents, predators, and scavengers. Pieces of skeletal debris may accumulate on the sea floor, later to become buried and lithified through compaction and cementation.

Describe the two different kinds of chert. How are they similar? How are they different?

All chert is composed of microcrystalline (cryptocrystalline) quartz. Biochemical chert is derived from the siliceous skeletons of micro-organisms (diatoms and radiolarians), which deposit in vast layers on the sea floor after the death of the organisms. Chert also occurs as a replacement mineral; groundwater may dissolve portions of limestone (when groundwater is undersaturated with respect to calcite) and fill in the resultant void space with microcrystalline quartz (with respect to which the groundwater is saturated). Bedding (layering) is absent in replacement chert.

Compare the deposits of an alluvial fan with those of a deep-marine deposit.

Alluvial fans are wedge-shaped deposits occurring at the foot of an eroding mountain range. The sediments are typically coarse (sand, pebbles, or cobbles) and contain substantial amounts of feldspar (physically weathered from a typically granitic montane source). Deep-marine deposits are dominated by the skeletons of planktonic microorganisms (chalk derived from foraminiferans, and bedded chert derived from diatoms and radiolarians ) and clay (which settles to form finely laminated mudstone).

What are cross beds, and how do they form? How can you use cross beds to read the current direction?

Cross beds form from sediment deposited on the lee (down-current) side of dunes and ripples. These angled beds dip downward in the down-current direction (parallel to the lee side of the dune or ripple).

How does dolostone differ from limestone, and how does dolostone form?

Dolostone is composed of the mineral dolomite, (Ca, Mg)CO3, whereas limestone is generally composed of calcite, CaCO3. Dolostone contains more magnesium (a trace impurity in natural calcite) and has a different crystalline structure. Dolomite can form through chemical alteration of limestone by reaction with groundwater bearing magnesium.

Describe how clastic sedimentary rocks form from its unweathered parent rock.

First, physical and chemical weathering break up and alter the parent rock to form detrital fragments of parent material, dissolved ions, and clay. This sediment of weathered grains is then eroded from the parent surface and transported away from the source by water, wind, or glacial ice (or directly by gravity, in the case of large clasts on a slope). Ultimately, the sediment will settle out of the transport medium. Eventually, loose grains of deposited sediment may become buried under additional sediment, compacted, and cemented to form sedimentary rock.

Describe the difference between lithification and diagenesis.

Lithification is the transformation of loose sediment into intact sedimentary rock via compaction and cementation. Diagenesis is the process (chemical, biological, or physical) by which any sedimentary rock is produced.

How do grain size and shape, sorting, sphericity, and angularity change as sediments move downstream?

Mechanical forces such as tumbling and abrasion wear on sediments as they are transported downstream. Angular protuberances are especially likely to be broken off. As a result, grain size decreases, with grains becoming more spherical and more rounded (less angular). The speed at which the water in a stream is traveling decreases along its course, and the capacity of the stream to carry sediment is directly related to its rate of flow. So larger grains are deposited upstream from finer grains (grains become sorted as they travel downstream).

Do all sedimentary rocks have same composition?

Sedimentary rocks have diverse compositions. Evaporite formation requires a fluid with dissolved ions (typically seawater) to be evaporated to such an extent that the ions will precipitate out, typically as halide, carbonate, or sulfate minerals.

Describe how a turbidity current forms and moves. How does it produce graded bedding?

Turbidity currents form when sediment becomes unstable on a subaqueous slope and tumbles downward, pulling a current of water with it. After a while, the velocity of the turbidity current slows. The heaviest (largest) particles settle out first, whereas smaller, lighter particles stay in suspension for longer periods. Ultimately, these smaller particles settle atop the coarser grains to produce graded bedding, a grain-sized gradient from coarse (near the base of the bed) to fine (at the top of the bed).