Wooster Geologists in Jamaica May 2001 (Page 3)

Just inland of Rio Bueno, cliffs of the Hope Gate Formation (Middle Pleistocene) are prominent above a terrace. The base of these cliffs shows an intertidal erosional notch cut during the deposition of the Falmouth, marking its highest extent. The Hope Gate has since been extensively dolomitized, so it is easy to distinguish it from the still aragonitic and calcitic Falmouth. If you look closely at this cliff you can see molds of corals which were present in the Hope Gate prior to diagenesis.

The ancient "Falmouth" erosional notch cut into the underlying Hope Gate Formation (Middle Pleistocene) as exposed near Rio Bueno. This notch was eroded into the rock during the Falmouth highstand, and is thus now well inland. A larger version of this image is available.

The eroded top of the Hope Gate Formation and bottom of the Falmouth at the east headland of Rio Bueno Harbour ("Overlook House"). The point of the hammer marks the top of the Hope Gate dolomitized limestone and the bottom of encrusting corals (Montastrea annularis) of the Falmouth. This is thus an ancient "rocky shore" exposure. A larger image is available.

At the "Overlook House" exposure on the eastern headland of Rio Bueno Harbour is an excellent view of the Hope Gate - Falmouth contact. Falmouth corals encrust the bored surface of the Hope Gate, especially where it has relatively high relief. At this location we can see a small Montastrea annularis reef which grew from the Hope Gate. Pieces of similar corals form a rubbly unit shoreward from this Hope Gate protrusion. The Hope Gate here may have formed a wave-resistant barrier, allowing Falmouth corals to grow in a relatively low-energy environment behind it.

On this Hope Gate erosional surface the large borings of bivalves (Gastrochaenolites) and smaller holes of sponges and worms are quite evident, although they are difficult to distinguish from later boring associated with the sea level decline at the end of Falmouth deposition. This Eemian rocky shore can be compared with dozens of contemporaneous examples around the world.

The hammer head again marks the boundary between the lower Hope Gate and the onlapping Falmouth Formation at Overlook House. Note the large Gastrochaenolites borings visible in the moss-covered portion of the Hope Gate. A larger version of this image is available.

Jerome Hall is standing on the top of small patch reefs within the Falmouth Formation; Sara is sitting near the top of the underlying Hope Gate Formation. The surface Jerome is standing on appears to be truncated. The Falmouth above his feet is almost entirely broken pieces of coral (primarily Porites), shells and carbonate sand. This is some type of event horizon. This exposure is also near the Overlook House. A larger image is available.

Behind the Hope Gate protrusion at Overlook House are several coral patch reefs, most dominated by Montastrea, Diploria, Acropora and Porites. They appear to have grown in protected waters behind the rocky ridge with its mantling of corals. These reefs, though, have flat tops as if they were all killed and truncated at the same time. On top of the reefs is a thick deposit of sandy coral debris rich in Porites and numerous mollusks. The base of this debris horizon is roughly coincident with the top of the Hope Gate rocky protrusion. In places where the patch reefs are not present, their place is taken by Montastrea and Diploria debris. Even here the Porites-debris layer appears to truncate the sediments below. We suspect that this layer represents a significant event horizon, but that it is not related to a eustatic change. Maybe it was formed during a massive hurricane? The top of this sequence ends in a "caliche cap rock", so we cannot determine if the erosional surface at Richie Richie's is present here.

We hope to describe the paleoecological changes associated with the Falmouth transgression over the Hope Gate, and try to deduce what this major intra-Falmouth horizon may represent.

Jerome's feet, just visible near the center of the photograph, are again on top of apparently truncated patch coral reef. Andrea is standing near the upper surface of the underlying Hope Gate. Overlook House. A larger version of this image is available.

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