Man Head Cay Project

Man Head Cay is an island in Rice Bay, San Salvador, Bahamas. It is Pleistocene in age, and consists mostly of crossbedded calcareous sandstone (skeletal grainstone) of eolian origin.



Photo of Man Head Cay from Rice Bay Beach, facing Northeast.

Man Head Cay is stratigraphically problematical. Carew and Mylroie (1995) believe it is 125,000 years old (oxygen isotope substage 5e), but Hearty and Kindler (1993) believe it is 80,000 years old (oxygen isotope substage 5a).

I analyzed 8 thin sections from Man Head Cay for my thesis. The samples were taken from the vicinity of column 2 (see below). I found that the grains consist of skeletal fragments, peloids, and intraclasts, with high porosity (26%) and freshwater, grain-skin cement, with no ooids, oolitic clasts, or mud. A pie chart shows the composition of Man Head Cay.

Dr. Cindy Carney, Dr. Mark Boardman, and I organized this project to be completed by students taking Carbonate Depositional Systems, a course they run every year in late June. The goal of the project was to obtain a series of rock columns from points around the cliffs of the island, in order to have a large number of trained eyes looking closely at the rock in the short amount of time that could be used to do so. First (June 24, 1999), a reconnaisance mission was carried out by Michelle Geissler and I (Michelle's thesis area is Rice Bay also), in order to take photographs and flag out locations of where the rock columns were to be drawn. We selected 8 sites, all on the southwestern (leeward) side of the island. No columns were drawn on the northeastern side due to higher wave energy, inaccessibility, and the presence of agressive birds. Some interesting photographs were taken on the northeastern side, however. The main project (when the columns were drawn) was carried out on June 26, 1999.

Above is an aerial photograph of Man Head Cay, showing the locations of the eight rock columns (numbered 1 through 8). Location 9 is where 3 other photographs were taken. Click on the X's to see photographs of the outcrops along with their columns.

I, with the help of Alfred Sidman, set up measuring tape at the top of the cliff at each station, and generally tried to keep everyone doing the same thing the same way. Dr. Carney and Dr. Boardman came out towards the end of our time there to look around and answer questions.

I acted as editor and drew all of the columns using Adobe Photoshop using the descriptions of the columns provided by the students.

Important results from the columns suggest that the protosol seen clearly at location 9 is not identifiable on the southwestern side of the island, and that no facies other than eolian facies are present anywhere on the island.

Update on Man Head Cay following the 10th Symposium

At the 10th Symposium on the Geology of the Bahamas and Other Carbonate Regions, held at the Bahamian Field Station on June 8-12, 2000, I led a field trip to Man Head Cay with 9 other geologists. Danielle Foye (of the Ohio Dept. of Natural Resources, Lake Erie Geology Group) and I examined the protosol seen at Location 9 above. We were able to follow the protosol to the northeast, where it joins the paleosol on the upper surface of the island. The protosol does not outcrop anywhere else on the northeastern (ocean) side of the island. We also followed the protosol around the southern tip of the island, where it is approximately 2m below the paleosol. As we tried to follow it around to the southwestern side of the island from there, we saw that the protosol was covered by talus. On the other side of the talus pile the protosol was not identifiable upon close examination of the nearest outcrops. Also, no marine signature (e.g., coral rubble) could be seen anywhere along the protosol.

A photomosaic of the southeastern side of Man Head Cay in the vicinity of the protosol is shown. The photo represents about 150 deg. of arc, so the rightmost side is about due south, while the leftmost side is northwest. A barely visible human figure (Dr. Sally Walker) squats at the protosol on the left. The protosol can be seen to pinch off with the overlying paleosol by following it to the right from the figure.

These outcrop relationships suggest to me that Carew and Mylroie's (1995) interpretation of the protosol on Man Head Cay is correct. That is, the protosol represents only a pause in the period of deposition that led to the formation of Man Head Cay. Their interpretation of Man Head Cay is that it is a regressive phase eolianite of the Oxygen Isotope Stage 5e highstand. So, a pause in the deposition could be caused by a pause, or even a short reversal, in the regression.

Of course, Man Head Cay may not have been deposited during the Oxygen Isotope Stage 5e highstand. Why not the Stage 7 higstand? Or the 5c or 5a highstands? Carew and Mylroie mapped Man Head Cay as the Cockburn Town Member of the Grotto beach Formation (that is, the Stage 5e highstand) due to the presence of a single paleosol on the surface. If there were two paleosols, the island would be mapped as the Owl's Hole Formation (Stage 7 or older). But could the single paleosol be a composite paleosol? Closer examination of the paleosol all around the island is needed to answer this question based on outcrop relationships.

But there may be another way to answer this question. Dr. Bruce Panuska has begun doing magnetostratigraphy on San Salvador in the last few years, and has identified several magnetotypes there. He expressed interest in taking samples from Man Head Cay, perhaps during the winter of 2000-1. He may be able to correlate its magnetic signature with other parts of the island that may have been dated or shown to correlate with other parts of the island using other methods. The results may confirm placement of Man Head Cay in the Grotto Beach Formation or may suggest that the island is much older and perhaps should be placed within the Owl's Hole Formation.

James Stuby, July 9, 2000.

stubotics@hotmail.com




Copyright 2006 James L. Stuby. All Rights Reserved.