Buck Island Reef: 70 million years, in one small ridge

Buck Island is a single east–west ridge off the north-east coast of St Croix:
1,830 m long, 670 m across at its widest, rising to 104 m. Ninety per cent of
it is steeper than a 30% slope. There are two flat places with sand — Diedrichs
Point on the south side and West Beach at the western end.

Around it is the reason the monument exists: an emergent linear reef with some
of the finest coral gardens in the Caribbean, and an interpretive snorkelling
route through it — the underwater trail.

Seventy million years are on display here, from Upper Cretaceous
deep-marine sediment to reef that is still growing.

The island is older than the reef by a very long way

The island is Caledonia Formation, laid down between 99.6 and 65.5 million
years ago as sediment at the foot of an island-arc volcano, in deep water, and
later reworked by deep-ocean currents.

Since then it has been pushed eastward, squeezed upward and tilted into the
position it now holds. The formation is exposed along most of the coast except
the western end, and Holocene beach rock — consolidated carbonate sand — rings
much of the island.

The reef is 7,700 years old

It began forming on a pre-existing carbonate platform when sea level was
lower. That platform is limestone laid down by an older reef, which stood
exposed to the weather through the Pleistocene ice ages.

The modern reef is dominated by **elkhorn coral, Acropora palmata, which is
a threatened species.** Around it the monument holds a range of benthic
habitats: coral reef, hardbottom, unconsolidated sediment and submerged
vegetation.

Three things acting on it now

Sea level rise is the significant one, for the coastal processes, for the
reef and the other habitats, and eventually for the beaches and the underwater
trail people come for.

Hurricanes cut both ways. Storm waves scour and pound reef down to pavement
and rubble; surge undercuts beaches, shifts their profile — which matters to
nesting turtles — and drops sediment on fragile coral. The same storms recycle
nutrients quickly and sweep away excess sediment that would otherwise smother
the reef.

Earthquakes. The Caribbean is seismically active — earthquakes, submarine
slides, submarine volcanic eruptions, pyroclastic flows, tsunamis. Buck Island
sits between the Greater and Lesser Antilles at a tectonic hinge, where plate
motion changes from transform (strike-slip) to convergent (subduction). The
USGS monitors it from a station in Puerto Rico.

Fossils, and where they are not

The fossils here are in the reef, not in the Cretaceous rock the island is
made of — although the Caledonia Formation does carry fossils elsewhere, so
future fieldwork may well find some.

Hubbard and colleagues logged seven cores from the main reef and two from Buck
Island Bar to the north. All but one went through the whole Holocene reef and
into Pleistocene strata beneath. The fossil corals in them include
A. cervicornis, Diploria spp., Millepora annularis and
Porites astreoides.

All NPS fossil resources are
protected under the Paleontological Resources Preservation Act of 2009.

Further reading

The Geologic Resources Inventory report and the Soil Resources Inventory for
the monument are both published by the National Park Service.

Sources

Written from the National Park Service's Geodiversity Atlas entry for Buck
Island Reef National Monument, a work of the United States government in the
public domain. Reef chronology and cores from Hubbard et al. (2005); elkhorn
coral and island description from Bythell et al. (1989); Caledonia Formation
from Whetten (1966); paleontological inventory from Toscano et al. (2010).