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A cache by acanthaster Message this owner

Hidden : 1/15/2010

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Geocache Description:

Visit Coral Bones to learn some Grand Cayman geology and appreciate how this beautiful island came into existence. At Coral Bones you step back in time – a time when sea level was higher, coral was king, and man was no where to be seen. The modern coral reefs attract tourists here to take pictures and keep memories, but the ancestral reefs, frozen in stone and visible here, have another story to tell. (Also a cool area to pet turtles and dolphins and grab a virtual.)

Overview

At the Coral Bones site, or any place on one of the Cayman Islands, for that matter, you will actually be standing on a mountain top. That's right, pull the plug on the ocean (or drop sea level by forming unbelievably massive polar ice caps) and you would see yourself surrounded by a steep and deep valley to the south and a broad plain to the north. Look east or west you'd see that you are standing on one of the peaks of an east-west trending chain of mountains, today called the Cayman Ridge. If it is a clear day on your mountain top you might even see another mountain peak, a little higher, about 70 miles to the east. That would be Little Cayman. Just beyond Little Cayman would be an even higher peak - Cayman Brac. Way off to the west, looking along the ridge, you see yet another peak. Today this peak is called Misteriosa Bank. This bank is not an island because it didn't quite make it to the top, rising just short of current sea level by only about 60 feet.

The cool thing about this mountain chain is that it was formed by hot volcanic rock that built up along the margin of tectonic plates. The crustal movement and volcanic buildup pushed the submarine mountains high enough near the warm ocean surface and sunlight that limestone (calcium carbonate) secreting organisms like algae, corals, and clams could start the cycle of life, reproduction, and death. Over the past 40-50 million years of sediment deposition and erosion (caused by many sea level changes), this piled up thick sequences of limestone and dolostone (chemically altered limestone) that form the Cayman Islands today.

At the Coral Bones site, the youngest fossil organisms, particularly corals, can be easily seen and recognized. These fossil corals are about 120,000 years old, and form the Ironshore Formation, upon which the enticing limestone sands of the western half of Grand Cayman sit today. Just offshore, modern coral communities identical to the fossil assemblages in the Ironshore Formation continue to build up the limestone sediments. Aren't you glad they do?

And now, here's just a little more detail for any geocaching rockhounds.

Tectonics

The Cayman Islands form limestone and dolostone caps on mountain peaks along an east-west trending submarine mountain chain called the Cayman Ridge (Figure 1). The Cayman Ridge is the remnant of a volcanic arc that formed in a subduction zone in Eocene time 50 million years ago. Today this zone forms an active transform fault, where the southern margin of the North American Plate slides to the west along the northern margin of the Caribbean Plate. A small spreading center that drives plate movement occurs just southwest of Grand Cayman (Figure 1). The transform fault, where the sliding action is still occurring, is in the 20,000 foot deep Cayman Trench, the deepest waters of the Caribbean. Evidence that this fault system is still active is the terrible earthquake that hit Haiti in January 2010. Tremors were felt in many areas along this fault system, including the Cayman Islands.

The Cayman Formation

The Miocene age Cayman Formation, part of the larger Bluff Group, is exposed over most of the eastern end of Grand Cayman (Figure 2). This 10 million year old formation is a dolomitized mudstone (fine grained) and wackestone (fine grained matrix with floating fossils of algae, small corals, and foraminifera). Dolomitization is a mineral change process that occurs after the limestone (calcium carbonate) is deposited, and then about half of the calcium in the mineral matrix is replaced by magnesium from the seawater, to form dolomite.

Depositionally, there is no evidence of coral reef development in the Cayman Formation, but the diverse fossil assemblage, including fragments of the branching coral Stylophora, indicates an open marine environment - perhaps on a shallow bank less than 30 m (100 feet) deep.

Stop by the East End Lighthouse to see a great example of the Cayman Formation (and search for a geocache here). Climb the steps to the lighthouse and notice all the shallow potholes in the rock (check out Figure 10). This is an example of karstification, where slightly acidic rainwater dissolves some of the dolostone. This is how caves form, and Grand Cayman has a number of caves to visit. That's what made it so popular with booty hiding pirates. In fact, most of the island has been karstified, making it more like a giant sponge than solid rock. Even during heavy rains the rainwater readily disappears into the many pore spaces of the rock, forming a freshwater lens that sits atop the more dense seawater. The highly porous rock is why Grand Cayman has no rivers, which in turn reduces island sediment runoff and preserves the mid-oceanic water clarity that visitors enjoy

The Pedro Castle Formation

Following deposition of the Cayman island formation, sea level dropped at the end of the Miocene to a level about 40 meters below current sea level. This lowstand lasted for about 1.5 million years, leading to an erosional surface called the Cayman unconformity. When sea level rose again in the Pliocene (to about 12 meters above current levels), the limestone factory of tropical organisms kicked back into gear and deposited the Pedro Castle Formation on top of the irregular unconformity. Along with the Cayman Formation, the Pedro Castle is also part of the Bluff Group. On some parts of the island the Bluff is over 1200 feet thick. Similar to the Cayman Formation, the Pedro Castle is a dolomitized mudstone and fossiliferous wackestone, containing fossils of algae, coral, echinoderms (e.g., sea urchins) and clams. The Pedro Castle formation is exposed on only a small wedge on Grand Cayman between Savannah and Boddentown (Figure 2). At the end of the Pliocene, sea level dropped again and stayed down for about 2 million years, exposing and eroding much of both the Pedro Castle and the Cayman formations.

The Ironshore Formation

Then sea level, as it always does, rose again above its current level about 125,000 years ago, providing the opportunity for Pleistocene coral reefs to form on a number of islands in the Caribbean (including the Florida Keys). The limestone of the Ironshore Formation, which underlies most of the western half of Grand Cayman Island, is a complex of shallow water limestone environments ranging from quiet water mudstones to high wave energy grainstones and coral reefs (called boundstones). The most recognizable organisms forming portions of the Ironshore Formation around the margins of Grand Cayman are the massive head corals and coral fragments (Figures 4, 5, and 7). Coral species recognizable in the fossil reefs are the same ones that form the modern reefs today in the Caymans. Some of the more common ones include brain corals (Diploria), star corals (Monastrea), and the elk horn and stag horn branching corals (Acropora). Brain corals are some of the most obvious preserved fossils in the Ironshore Formation, and look for these at the Coral Bones earthcache site and along other rocky shoreline areas (Eden Rock near Georgetown and the blow hole on the south side of the island are a few of many). If you make it to the blow hole on south shore, be sure to record the Thar She Blows earthcache.

**Questions required for logging CORAL BONES:**

(Send answers to me by e-mail via this site and DO NOT post on your log or it must be deleted to prevent spoilage.)

1. Look at some of the head corals (large, generally round) and let me how big they appear to be (such as baseball, grapefruit, basketball, beach ball, larger?).

2. What do you think is the function of the man-made trench at this site?

3. Tell me what other living critters you see crawling on or attached to the fossil rocks.

Bonus: You don't have to, but it would be cool if you took a picture of yourself or your GPS with the Ironshore rock. Post it on the log.

Key References

Rigby, J. K., R. H. Roberts. 1976. Geology, reefs and marine communities of Grand Cayman Island, British West Indies. In: Geology Studies Special Publication No. 4: Grand Cayman Island, Geology, Sediments, and Marine Communities (edited by J. K. Rigby and E. Shaw), pp 1-95. Department of Geology, Brigham Young University, Provo UT, USA, 122 pp.

Brian Jones, K.C. Ng, and I.G. Hunter. 1997. Geology and Hydrogeology of the Cayman Islands. In, Developments in Sedimentology #54 : Geology and Hydrogeology of Carbonate Islands. Vacher and Quinn, eds. Elsevier, 1997.

Unruh, Matt. 2008. The Cayman Islands: A Geological View. http://www.emporia.edu/earthsci/student/unruh1/webpage.html

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Geocache Description:

Overview

Tectonics

The Cayman Formation

The Pedro Castle Formation

The Ironshore Formation

**Questions required for logging CORAL BONES:**

Attributes

Inventory

Bookmark Lists

Coral Bones EarthCache

This cache has been archived.

Watch

Geocache Description:

Overview

Tectonics

The Cayman Formation

The Pedro Castle Formation

The Ironshore Formation

Questions required for logging CORAL BONES:

Attributes

Inventory

Bookmark Lists

Treasures

**Questions required for logging CORAL BONES:**