GY 111 Lecture Notes D Haywick 2008 09 1 GY 111 Lecture Note Series Sedimentary Environments 4 Carbonates Lecture Goals A Shelf Terminology B Reefs atolls and oolites C Evaporites and evaporate basins Reference Press et al 2004 Chapter 7 Grotzinger et al 2007 Chapter 5 pages 108 120 127 GY 111 Lab manual Chapter 3 A Shelf Terminology So far in the sedimentology component of GY 111 we have focused on siliciclastic sedimentary environments that were restricted to terrestrial or shoreline areas e g alluvial fans rivers deltas beaches Today we start to get our feet wet Continents are surrounded by relatively shallow water These are the continental shelves and they are remarkably inconsistent in width In the Gulf of Mexico near were we live the shelf is very wide almost 80 miles in fact It is also wide along the eastern seaboard of the United States In contrast the shelf along the western seaboard of the country e g California Oregon is narrow in some places only a few miles wide The reason is that the west coast is a tectonically active region and the Gulf of Mexico eastern shoreline is not We call it a passive continental margin which more or less means boring at least as far as volcanoes and earthquakes are concerned Wide shelves occur along coastlines that are tectonically inactive which are areas where sediment has been being deposited for a long long long time The shelf is officially defined as the region surrounding continents where the water ranges from 10 m1 to 150 or 200 m in depth The end of the shelf is marked by a relatively sudden increase in slope at the shelf break from near 0 on the shelf to 3 along the continental slope The deepest part of the ocean the abyssal plain like the shelf is relatively flat but here is water is incredibly deep 5000 m or more Sedimentation on most shelves is restricted to the zone nearest the shoreline The rest of the shelf may be devoid of any recent sediment These areas are considered relict because the last time they received sediment was during the height of the last ice age approximately 18 000 years ago 1 You will recall that the nearshore zone which includes beaches extends outward from the shoreline to the place where the water depth reaches 10 m GY 111 Lecture Notes D Haywick 2008 09 2 At this time sea level was much lower 150m lower and what is today s continental shelf was a tree covered plain Sea level change is the single most important control of shelf sedimentation B Reefs and atolls Siliciclastic shelves are areas where vast quantities of sediment can accumulate In areas like Mobile where large rivers flow into the sea sediment that makes it onto the shelf is going to be dominated by siliciclastic grains remember these are the products of weathering and rivers transport these materials away from the source In areas where rivers are not as abundant or the topography is flatter e g the Florida Peninsula siliciclastic sediment may not accumulate in significant amounts on the shoreline or on the shelf In these areas other sediments like biochemical remains shells etc may be the dominate deposit Most of the biochemical sediment that you will see in GY 111 is composed of calcite which everyone will instantly recall has the following chemical composition CaCO3 There is another important carbonate mineral that makes up biochemical sedimentary rocks Aragonite has the same formula as calcite CaCO3 but a different crystal structure From our lectures on minerals earlier in the course you will realize that aragonite and calcite must be polymorphs like graphite and carbon You won t be able to distinguish aragonite from calcite in GY 111 labs but then again you won t have to All you have to do is to be able to identify biochemical sedimentary rocks Since they are comprised of CaCO3 minerals the easiest way to identify them is with the acid test If the rock fizzes it contains calcite or aragonite If the entire rock fizzes and therefore must be entirely composed of one or both of those carbonate minerals If it does you know it is biochemical in origin More importantly that rock is called limestone There are several types of limestone which you will see in the lab In the lecture component of GY 111 it is best to restrict ourselves to major types of limestones Let s start with one of the most impressive types that is associated with one of the most diverse ecosystems our planet has reefs see picture at start of this section The term reef has many different meanings depending upon who you are If you are a boater reef means anything underwater that can put a hole in the bottom of your boat if you hit it e g shallow rocks The geological and biological definition of GY 111 Lecture Notes D Haywick 2008 09 3 a reef is much more specific It is any framework of organisms normally composed of calcite and aragonite that grow in shallow marine normally tropical environments that cement themselves to the seafloor It is the latter part of that definition that will result in a hole in your boat if you hit one Reefs have been on this planet for an incredible long time Diverse one with hundreds of different calcium carbonate producing beasties have been around for over 500 million years the image to the right is a reconstruction of a 350 million year old reef complex that I photographed in a museum Less diverse ones that are still clearly reefs have been around for more than 2000 million years 2 BILLION years Today we frequently call reefs coral reefs in recognition of one of the more important framework builders Coralline algae not to be confused with brown yucky algae that cover tidal pools is another important reef builder Every student should witness the beauty and complexity of a tropical reef once in their lives I suggest a dive or snorkel over the Great Barrier Reef of Australia or the 2nd largest barrier reef complex in the world offshore of Belize Can t afford the trip down under then drive down to the Florida Keys When you witness a living reef and you see the corals fish and other beasties that inhabit it you realize that there is a lot of magic in this world More importantly reef ecosystems are completely separate from those in the terrestrial world like us We could kill ourselves off and they wouldn t care Sadly however human activities do impact reef ecosystems Pollution over fishing climate change and boaters putting holes in the bottom of their boats all damage or kill the corals that make up a good chunk of the reef So I guess you should