402-pp24 - UNIVERSITY OF SOUTH ALABAMA GY 402: Sedimentary...

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Unformatted text preview: UNIVERSITY OF SOUTH ALABAMA GY 402: Sedimentary Petrology Lecture 24: Reefs Last Time Shelves Definition and a bit of history Factors controlling shelf sedimentation Carbonate Shelves Continental Shelves By definition: shelves are: "oceanic environments characterized by moderate water depths (10m to 150/200m) in which a variety of shallow marine and moderate depth facies occur" http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/SedStrat%207/SedStrat7.html http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/SedStrat%207/SedStrat7.html Continental Shelves We currently recognize 6 types of shelves (5 of which are "depositional"). 1.Siliciclastic/detrital (sand, silt, clay) 2.Biogenic (carbonate sediment, shells, etc.) 3.Residual (in situ weathering of rocky substrates) 4.Volcanic (recent volcanic rocks) 5.Authigenic (chemical precipitates like glauconite, phosphate, dolomite) 6.Relict (which we will now ignore) Time does not allow us to examine all these types of shelves, so we will only consider the first 2. Factors influencing shelf sedimentation Six factors are recognized as influencing shelf sedimentation. The last three are designated as "biggie" factors. 1) Sea level fluctuations (controls distribution of relict sediments, barrier reefs etc.) 2) Animal-sediment interactions (modifies the substrate) 3) Chemistry (produces authigenic minerals) 4) Type and rate of sedimentation 5) Climate 6) Type and intensity of shelf hydraulic regime ("energy") Biggies Continental Shelves Hydraulic regime Three dominant processes are responsible for transporting, reworking and sorting sediment on shelves (including relict areas). 1) Tide-dominated shelves (17% of pericontinental shelf areas) 2) Ocean current-dominated shelves (3% of pericontinental shelves) 3) Storm-dominated shelves (80% of pericontinental shelf areas) We will only consider tide- and storm-dominated shelves Tide-dominated Shelves Macrotidal environment (+/- 4m tides) Induces strong bidirectional currents Produces elongated bedforms (ridges) From Reading (1975) Storm-dominated Shelves HCS (hummocky cross-stratification) From: Harms et al. (1975) From Walker and James (1992) Remember HCS? http://course1.winona.edu/csumma/images/sedstrux/hcs8a.jpg Temperate vs Tropical Carbonate Shelves http://www.pal.uni-erlangen.de/uploads/pics/cwcabb3.jpg http://www.pal.uni- Chalk Board Carbonate Shelves Area: 700 x 300 km Depth: 0 m to 200m (mostly < 3 m) Tongue of Ocean: 3500m deep Mesotidal (2 to 4 m tidal range) Also storm-dominated 3.5 to 4 m of Recent sediment Sedimentation rate 1000mm/1000yrs From Bathurst (1975) Carbonate Shelves The Grand Bahama Bank can be divided up into a series of facies. The type depends on your discipline. Lithofacies 1) reef (reef + coralgal) 2) oolitic (oolite + grapestone) 3) mobile oolite 4) mud From Bathurst (1975) Today's Agenda Reefs Types of reefs Reef facies/characteristics Reef growth Coral Reefs Reefs are biochemical sedimentary rocks consisting of corals and other calcite and aragonite-secreting organisms Coral Reefs They come in several varieties: 1) Barrier 2) Fringing 3) Pinnacle 4) Atolls Coral Reefs Atolls are isolated coral reefs that form on shelves or in much deeper water Coral Reefs Atolls are isolated coral reefs that form on shelves or in much deeper water Coral Reefs Usually have lots of in situ framework facies... Coral Reefs James, N.P., 1984. Reefs. In R.G. Walker, (ed) Facies Models (2nd ed). Geoscience Canada Reprint Series 1., 317p 1., Coral Reefs James, N.P., 1984. Reefs. In R.G. Walker, (ed) Facies Models (2nd ed). Geoscience Canada Reprint Series 1., 317p 1., ...but with lots of "holes" in which to find sediment Coral Reefs Reefs are a type of sedimentary environment and as such, are characterized by distinct facies Basic Reef Facies James, N.P., 1984. Reefs. In R.G. Walker, (ed) Facies Models (2nd ed). Geoscience Canada Reprint Series 1., 317p 1., Coral Reefs But as "biologically-produce facies", expect to see bioinfluences on facies character (e.g., coral morphology zonation) James, N.P., 1984. Reefs. In R.G. Walker, (ed) Facies Models (2nd ed). Geoscience Canada Reprint Series 1., 317p 1., Coral Reefs James, N.P., 1984. Reefs. In R.G. Walker, (ed) Facies Models (2nd ed). Geoscience Canada Reprint Series 1., 317p 1., Coral Reefs Coral reefs (e.g., GBR) also record sea level changes particularly with respect to mineral variations James, N.P., 1984. Reefs. In R.G. Walker, (ed) Facies Models (2nd ed). Geoscience Canada Reprint Series 1., 317p 1., Chalk Board Coral Reefs Past studies suggest that reefs go through 4 stages of growth linked to sea level change: James, N.P., 1984. Reefs. In R.G. Walker, (ed) Facies Models (2nd ed). Geoscience Canada Reprint Series 1., 317p 1., Coral Reefs Lastly, reefs have not always been part of the rock record James, N.P., 1984. Reefs. In R.G. Walker, (ed) Facies Models (2nd ed). Geoscience Canada Reprint Series 1., 317p 1., Upcoming Stuff Homework 1) Skeletal/Temperate thin section lab due Thursday 2) Moscow Landing introduction and methodology due Friday Lab this Week Skeletal/Temperate Limestones Next Lecture: Sea-level and sedimentation (New Zealand) ...
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This note was uploaded on 02/05/2012 for the course GLY 402 taught by Professor Staff during the Fall '11 term at S. Alabama.

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