Classifying and Naming Carbonate Sediments

Heather George Mark Weber Sarah Petty Daysi Nemecio Edgar Fennie Earth 515 11/3/15 Traveling to the Bahamas

1. Labeled “coarse fraction from North Pigeon Creek lagoon”: These are unconsolidated biogenic sediments. The white colored whole shells are mostly gastropods and bivalves and are granule to pebble sized. They are well sorted with no matrix or cement. Because of the size and the fact that the shells are intact, we interpret that these sediments are from organisms that lived and died in place. (North Pigeon Creek Lagoon) 2. Unconsolidated sediments from West Pigeon Creek tidal flats are biogenic- gastropod, algae and bivalve, with pisoids. They are yellowish grey in color, moderately sorted silt to pebble size grains with very few whole shells and broken pieces. There is some “quick” cementation, but it is weak and easily broken apart with fingers. The sediment size indicates a relatively low energy environment with occasional influxes in energy and local transportation of sediments. 3. Pigeon Creek Tidal Channel: Consists of pinkish grey, biogenic, angular, unconsolidated clasts. Size ranges from very fine sand grains to pebble. There is moderate sorting with both whole and broken shells fragments. The size, angularity and sorting are consistent with channel sediments. 4. Crust on pigeon creek tidal flats: biopackstone with platey structure, weakly consolidated, moderately well sorted. Fine sand size grains with some fragmented shells. The shell fragments consist of gastropods and algae is present as well. 5. Cockburn fossil reef: The Cockburn reef oobiograinstone is very pale orange, composed of carbonate shell fragments and ooids. The sediments are subrounded in alternating layers of fine sand, medium sand, and granules. The rock is carbonate cemented and the layers and grain sizes are consistent with a nearshore or foreshore environment. 6. Patch Reef Edge, Lindsey Reef: Sediments are white in color with some pink fragments. They are fine sand size grains to pebbles size fragmented coral and shells which are subrounded to angular, moderately sorted, unconsolidated, biogentic clasts. 7. Tom Hanna’s Beach Sand: The beach sand is white with pink shell fragments, coral, ooids, and pesoids. The sediments are very fine to coarse sand size grains, are sub-rounded to rounded, unconsolidated, and moderately sorted. 8. French Bay: Sand is well sorted, coarse-grained size, subrounded, consisting of ooids and pisoids. There are white, pink, and light orange granules with a porous layer. 9. Pleistocene Beach Sand: oograinstone is very pale orange, composed primarily of ooids but also shell fragments. The sediments are laid down in alternating layers of subrounded, very well sorted, medium sand sized grains and coarse sand sized grains. The rock is carbonate cemented with planar laminations. 10. Church Outcrop: Is white sediment of calcareous mud with a portion of black grey and light orange ribbons, black possibly from an algae. Grain size ranges from fine sand size grains to whole bivalve shells.

Subtidal Shelf/ Open Lagoons and Reef 1. Graham’s Harbor will have the most rapid exchange between shelf water and open ocean because it is the most exposed to the Atlantic ocean compared to Fernandez Bay. Fernandez Bay would be more restricted by the surrounding reefs and Cat Island Bank to the west. 2. Sediment pieces seem to be mostly shell and reef fragments with some whole shells 3. At Grahams Harbor subtidal environment, mean grain size is coarser than at Graham’s Harbor beach. Beach sediments are well sorted compared to the moderate sorting of the subtidal sediments. 4. Pieces of coral, shell fragments, and algae fragments collect around reefs. The reefs act as a barrier and filter to keep the shells, coral, and algae for the most part intact from the high energy waves. 5. The Bahamian shelf is much shorter and shallower than the California coast shelf. The California shelf along San Francisco is 20-50 m deep and up to 51 km away from the shore. The Bahamian shelf is not as deep and has a gentler slope of about 1-2 degrees. 6. Sediment size on Bahamian shelves is heavily influenced by reef structures. Reefs disperse wave energy before it reaches the shore, leading to a smaller sediment size on the beaches and very coarse sediment on the ocean side of the reefs. Along the California coast, water temperature is too cold for coral, and the lack of reef structures allow waves to break down the sediments.

Intertidal Beaches and Restricted Lagoons 1. The Cockburn reef oobiograinstone is very pale orange, composed of carbonate shell fragments and ooids. The sediments are subrounded in alternating layers of fine sand, medium sand, and granules. The rock is carbonate cemented and the layers and grain sizes are consistent with a nearshore or foreshore environment. Pleistocene Beach Sand oograinstone is very pale orange, composed primarily of ooids. The sediments are also laid down in alternating layers of subrounded, very well sorted, medium sand sized grains and coarse sand sized grains. The rock is carbonate cemented and the grain sizes are comparable to the grain types and sizes found in the Holocene Beach Sand deposits. These samples are different composition from those observed along the coast of California, where terrestrially sourced siliciclastic sediments will be more dominant than carbonate sediments. Along the California coast sediments will be larger and more well rounded than from San Salvador’s beach. 2. In the swash zone, sediments are very fine sand. Moving seaward, sediments first increase to coarse sand and granules, and then become finer grained, more rounded, and more well sorted from B-D. On the other side of the reef, sediments are coarse sand to pebble sized pieces of coral, bivalves, gastropods, and algae, indicating the highest energy environment. The reef absorbs most of the energy from waves in San Salvador before it gets to the beach. In contrast, the coarser pebble sized pieces of sediment would never be preserved in the higher energy environment and larger tidal range along the California coast. 3. Explain the variation in sediment type, size and sorting that you observe throughout the system: At the entrance of the delta, moderately sorted fine sand- granule sized sediments are very broken up with little to no whole shells, indicating a high energy environment. Sediments in the tidal channel are larger and slightly more poorly sorted, with more whole shells. Fine sand to pebble sized shell pieces and ooids are contained in the sample. In the tidal flats are moderately sorted silt sized to pebble sized bioclastic and ooid sediments. The smaller grain size range implies a lower energy environment (tidal flats). 6-20 mm intact pebble sized whole gastropod and bivalve shells are found in the pigeon creek lagoon. We interpreted that the shelled organisms lived and died in place. Well sorted fine to medium sized sand is found in the delta. Sediments in the delta are broken pieces of bivalves and algae, and ooids 4. Compare Pigeon Creek sediments to SF Bay sediments: The Pigeon creek sediments contain a very high proportion of shells and shell fragments. The sediments in pigeon creek are mostly carbonate of biogenic origin, and fairly angular. SF bay sediments are predominantly siliciclastic of terrestrial origin. The SF bay is deeper than the pigeon creek lagoon which will affect the processes that occur. The energy and tidal range is higher in SF bay, which results in coarser, more well rounded, and more well sorted sediments.

Supratidal and Terrestrial 1. The inland lake locations and salinity are primarily controlled by the permeability of the carbonate sand of the island. The location of the lakes closer to the ocean may have been lagoons at some point while the lakes farther inland are likely made by the water table seeping into low areas between paleo dunes of the same elevation as the water table and/or more porous sediment. Salinity of lakes decreases as you move inland because sea water can get to the outer lakes more easily through the sediment. 2. The dune sand is medium sized sand that is well sorted and subangular. The French Bay beach sand is coarse-grained subrounded sand that is also well sorted. Dune sand has lower energy (wind) than beach sand (foreshore) so is has finer grain size. This is the same kind of sorting expected to be seen along the California coast, the sources are different with the San Salvador Island being made of carbonates and the source of California’s beaches is siliciclastic. 3. The entire island is made from dunes. The reefs are broken down by waves and carried into the center where the carbonates are then cemented during low sea level episodes. These episodes can be connected to the O¹⁸ isotope amount as shown in Fig. 3 in the handout, which shows three such episodes. When the dunes become cemented they allow the island to grow outward. A map of the island with an overlay of dated dunes showing oldest in the middle of the island and younger dunes closer to the edges confirms the growth pattern of carbonate islands. California dunes are not cemented like San Salvador dunes are. 4. Paleosols are cemented in San Salvador, as shown by the hand samples. Iron is rarely seen in carbonates but only a little is needed to show oxidation. Soils in California are not cemented.

ENVIRONMENTAL IMPACT OF NEW DEVELOPMENT AND GLOBAL

CHANGES ON SAN SALVADOR ISLAND

The unique position of this island provides an excellent place for scuba diving among the other activities a Bahamian island has to offer. Businesses would like to exploit this aspect by making it a grand vacation destination, meaning an increase in human population and all the ‘comforts’ they would require. This island also has another unique aspect, it’s made of carbonate, which is important due to the dissolution characteristics of carbonate leading to a great concern for the future of the island. The greater importance of carbonate to San Salvador Island is due to the soil lost when the mahogany forests were removed for the slave plantations. The loss of the protective layer of soil and trees increased the area of exposed carbonate on the island allowing changes to have a larger impact than before. Just as carbonate dissolution by low pH is a concern should an increase in population occur, the fossil burning toys tourists would use results in an increase in CO2 emissions. CO2 lowers pH and this is of great concern to a carbonate island. Also, a larger population would be drawing more of the groundwater. This could result in a lowering of the water table causing salt water to infiltrate and contaminate wells making it difficult to sustain a large population on the island. Along with this, even though carbonate dissolution by high salinity has a very small effect on the rate of dissolution, should seawater infiltrate into the island enough it could become a large problem. Sadly even maintaining a small population may not be enough to help this island as increasing CO2 concentrations from the rest of the world in the atmosphere are partly absorbed by the oceans. This is globally changing the oceans pH and may cripple the growth of all carbonate islands on the planet.