GS 305- Sedimentary Geology
December 20, 1999
1. As we discussed in class, carbonate sediments primarily accumulate in two different depositional settings, either as epicratonic shelfal carbonate sequences, or as deeper marine oozes. Briefly elaborate as to exactly where carbonate sediment accumulates in each of these very different settings. Cratonic- Oceanic-
2. Banded iron formations (BIFs) span a specific interval of geologic time. In millions of years, what is this interval? Give me two dates, which mark the beginning (oldest deposits) and end (youngest deposits) of BIF accumulation. Why did BIFs only form during this interval of geologic time?
3. Bob Folk (1974, The natural history of crystalline calcium carbonate- the effect of magnesium content and salinity: Journal of Sedimentary Petrology, v. 44, p. 40-53) suggested that a first-order relationship exists between calcite Mg++ content and crystal habit. What is the nature of this relationship? Why does it exist?
4. Montañez and others (1996, Integrated Sr isotope variation and sea level history of Middle to Upper Cambrian platform carbonates- Implications for the evolution of Cambrian seawater 87Sr/86Sr: Geology, v. 24, p. 917-920) suggested that a first-order relationship exists between ocean water 86Sr/87Sr and sea level. What is the nature of this relationship? Why might it exist?
5. Purton and Brasier (1999, Giant protist Nummulties and its Eocene environment- life span and habitat insights from ?18O and ?13C data from Nummulites and Venericardia, Hampshire basin, UK: Geology, v. 27, p. 711-714) suggest a relation between season of growth and 18O composition of carbonate hardparts in Nummulties and Venericardia. What is the nature of this relationship? Why does it exist?
6. Sharp and Cerling (1998, Fossil isotope records of seasonal climate and ecology: straight from the horse’s mouth: Geology, v. 26, p. 219-222) suggest a similar relation between season of growth and 18O composition of teeth from a Miocene horse. What is the nature of this relationship? Why does it exist? (Hint- the correct answer is not the same as that in 5 above).
7. Cohen and Thouin (1987, Nearshore carbonate deposits in Lake Tanganyika: Geology, v. 15, p. 414-418) report on the nature of nearshore carbonate sediments in Lake Tanganyika. What is the tectonic setting of this lake basin? What is the dominant mineralogy and composition of these carbonate components. Cohen and Thouin describe at least 5 types of carbonate sediment in this region. Specifically name two.
8. Marshall and Ashton (1980; Isotopic and trace element evidence for sub-marine lithification of hardgrounds in the Jurassic of eastern England: Sedimentology, v. 27, p. 271-289) document the formation of "hardground" cements in one specific diagenetic environment. What is the diagenetic setting of hardground cement formation? Why do they think that this is the case?
9. Allan and Matthews (1982, Isotope signatures associated with early meteoric diagenesis: Sedimentology, v. 29, p. 797-817) document the 13C isotopic signatures acquired in the early meteoric diagenetic environment. What is the mineralogy and composition of cements formed in this diagenetic setting? What is the carbon isotopic signature of these meteoric cements?
10. What is the Mg++/Ca++ ratio of a fluid in equilibrium with calcite and dolomite at the Earth's surface. Show all calculations clearly.
11. Write a balanced equation for dolomitization of HMC at a low water/rock ratio (autochthonous Mg++).
12. Briefly define the following: Diamictite- Cap Carbonate- Interfluve- Paleopedology- Loess- C3 plants- Planktic- Benthic- Gyttja- Autochthonous- 18O enrichment- IAP-
13. List four of the requisite geologic features necessary for the accumulation of commercial deposit of liquid petroleum.
14. In the space below, write a generalized set of equations showing the dependency of carbonate solubility on pCO2.
15. Many lakes throughout temperate regions of the midwest (like Michigan) precipitate vast quantities of carbonate sediment known as "marl". With reference to the equation(s) you have just formulated, tell me why carbonate precipitation occurs in these lakes.
16. In the space below, draw me a cross section showing that you have a good grip on the concepts of “mixing zone” dolomitization.
17. Serious question. Tell me
something esoteric that you have learned in preparation for this exam that
I have neglected to cover in the previous questions.