Buck Island Reef National Monument Geologic Resources Inventory

Geologic Features and Processes
This section describes the most prominent and distinctive geologic features and processes
in Buck Island Reef National Monument.
Buck Island is made up of the Upper Cretaceous
Caledonia Formation (map unit symbol Kc) (fig. 14).
Surrounding the island are recent surficial deposits (Qal),
which Whetten (1966) identified as sand, beach rock,
and stream deposits. Uncolonized bedrock (hbub),
colonized bedrock (rcb), and colonized pavement (rcp)
also occur in the immediate vicinity of the island (figs. 15
and 16). “Colonized” is a descriptive term used in the
digital geologic map data for Buck Islands Reef National
Monument. For instance, macroalgae, hard coral,
gorgonians, and other sessile invertebrates colonize the
bedrock and pavement. Seagrass, at varying levels of
“patchiness,” grows primarily to the south of the island,
but patches also occur in the waters to the north of the
island (fig. 17).
A linear reef (rlr) surrounds the east end of Buck Island,
starting midway on the south side then wrapping
completely around the north side (see front cover photo
and fig. 18). A segment of linear reef also runs southeast/north-west
in the waters at the western end of the
island. Seaward of the main linear reef is a large area of
aggregated patch reef (rpra) (fig. 19). Surrounding and
seaward of the aggregated patch reef are colonized
pavement (rcp) (fig. 16), much with sand channels (rcpc),
and areas of scattered coral or rock in unconsolidated
sediment (rscr) (fig. 20).
Fossils are another geologic resource of interest at Buck
Island Reef National Monument. Paleontological
resources occur in the Caledonia Formation (Kc) and
fossil reefs, which are discussed in the “Paleontological
Resources” section.
Caledonia Formation
Buck Island is composed of the Upper Cretaceous (99.6
million to 65.5 million years ago) Caledonia Formation
(Kc). Although other formations are included on the
digital geologic map for the national monument (see
“Geologic Map Data”) and have contributed to the
regional geologic history (see “Geologic History”), the
Caledonia Formation is the sole bedrock unit at the
national monument. Beach rock (sand and gravel
cemented with calcium carbonate) might be thought of
as “bedrock,” but Whetten (1966) identified beach rock
as a surficial deposit and mapped it as part of alluvium
(Qal). With respect to the benthic-habitat categories, the
Caledonia Formation is mapped as land (l) by Kendall et
al. (1999).
The Caledonia Formation was named for the wellexposed
outcrops in the Caledonia Valley on
northwestern St. Croix. At the national monument,
exposures occur on the shores of Buck Island, in
particular the south shore.
About 70 million years ago, the Caledonia Formation was
deposited in a tectonically active, chain of island
volcanoes, called an “island arc.” The formation is
composed of volcaniclastic sediments from a
contemporaneous arc volcano, with lesser amounts of
mud, sand, limestone (skeletal remains of coral), and
chert.
Sediments were deposited via gravity at the base of a
slope of an island arc, specifically the lower slope apron.
Later, fluid-driven processes reworked the sediments via
deep-ocean bottom currents (Stanley 1989). According
to Stanley (1989), most layers of the Caledonia
Formation appear as intermediate between mass-flow
(turbidite) and bottom-current (contourite) deposits in
the rock record. Turbidites originate as turbidity
(density) currents, which can be generated by storm
waves, tsunami, earthquake-induced sliding, tectonic
movement, or an over-supply of sediment (Neuendorf et
al. 2005). In this deep-marine setting below wave base,
thermohaline (deep-water) circulation, not waves, drove
transport and sedimentation of contourites (Stanley
1989).
Geologists estimate that the sequence of events that
culminated in the Caledonia Formation deposited
3,000 m (10,000 ft) (Stanley 1989) to 5,500 m (18,000 ft)
(Whetten 1966) of sediment over 14 million years and
perhaps as long as 30 million years (Speed and Joyce
1989).
Benthic Habitats
In addition to land (l), or the Caledonia Formation (Kc),
Kendall et al. (1999) mapped the seafloor around Buck
Island, separating the substrate into a variety of benthic
habitats. The primary habitats are coral reef and
hardbottom, unconsolidated sediments, and submerged
vegetation (see “Geologic Map Data”). According to
Pittman et al. (2008), 78% of all benthic habitat at Buck
Island Reef National Monument is reef and hardbottom,
and 22% is unconsolidated sediment and submerged
vegetation.
Reefs
A coral reef is a dynamic, continually changing
environment resulting from by both constructive and
destructive processes. Construction directly depends on
the activity of carbonate-secreting organisms, namely
corals. Also, algae bind loose materials together,
strengthening the reef edifice. When coral and other
carbonate-secreting organisms die, their hard parts are
added to the outer layer of the reef structure, which
sustains the reef’s upward and lateral growth over time.
In this way, the dead marine organisms, which become
BUIS Geologic Resources Inventory Report 19