Geoscience Reference
In-Depth Information
The lower member of the Culebra Formation is only exposed in Section 1 (Figure
6), where it conformably overlies the L.C.F. and conformably underlies the E.L. The
L.C.F. consists of agglomerate with tuffaceous claystone interbeds representing sub-
aerial volcanism with periods of paleosol development. The lower member of the Culebra
Formation consists mostly of carbonaceous mudstone with thin tabular interbeds of
fossiliferous lithic wacke. The base of the lower member is defi ned by a black lignitic
mudstone bed that is overlain by calcarenite (calcareous sandstone) and pebble cal-
cirudite (calcareous conglomerate). The top of the lower member is defi ned by a very
distinctive bed of fi ne-grained, lithic wacke that contains pectinids (
Lepidopecten proterus
)
and spondylids (
Spondylus scotti
). The lower member of the Culebra Formation repre-
sents the beginning of a transgressive sequence, with paleosols developed in volcanic
sediments of the L.C.F. below the base of the Culebra Formation, and shallow-marine
facies in the Culebra Formation (Figure 11). Carbonaceous mudstone and lignitic in-
terbeds represent a shallow lagoon protected from the open ocean by a fringing reef
(E.L.), which is consistent with carbonized compressions of sea grass and wood. This
interpretation is also consistent with the presence of brackish Elphidium foraminifera
and ostracodes in the lower member described by Blacut and Kleinpell (1969) and
Van den Bold (1972), respectively. The presence of Elphidium and the absence of glo-
bigerinid foraminifera (the latter are common in siltstone in the upper member of the
Culebra Formation) suggest a current-protected, nearshore environment (Blacut and
Kleinpell, 1969). The lithic wacke interbeds represent storm deposits in the lagoon.
Burrows in the underlying mudstone were rapidly infi lled by sand during the storm
events, thereby preserving the trace fossil
Thalassinoides
sp. The lowermost calcaren-
ite and calcirudite beds within the carbonaceous mudstone represent bioclastic debris
likely derived from patches of coral in the lagoon (Johnson and Kirby, 2006).
The E.L. is the middle member of the Culebra Formation and consists of fi ve dis-
tinct facies in Section 1 (Johnson and Kirby, 2006). The base of the E.L., which over-
lies conformably the “pectinid-spondylid” sandstone bed in the lower member of the
Culebra Formation, is defi ned by a branching-coral boundstone containing abundant
Acropora saludensis and Montastraea canalis in a very fi ne-grained calcarenite matrix.
The second facies consists of white, rhodolithic limestone. The third facies consists of
another branching-coral boundstone dominated by
Acropora saludensis
,
Stylophora
granulate
, and
Porites douvillei
as part of a diverse assemblage of corals in a mud ma-
trix with isolated coral heads of
Montastraea imperatoris
in life position. The fourth
facies consists of platy-coral boundstone. The top of the E.L. is defi ned by the fi fth
facies, which consists of calcirudite containing fragmented corals in a calcarenite ma-
trix and displaced head corals of Montastraea species and massive Porites species.
The E.L. represents a fringing reef that protected a neighboring lagoon, as represented
by the carbonaceous mudstone facies in the underlying lower member (Johnson and
Kirby, 2006). The E.L. is overlain conformably by alternating beds of sandstone and
siltstone of the upper member of the Culebra Formation.
The upper member of the Culebra Formation consists of fi ve distinct facies and is
best exposed in Sections 1, 3-5, and 7 (Figure 6). The base is defi ned by alternating
beds of sandstone and siltstone that conformably overlie the E.L. in Section 1. The
lowermost sandstone bed contains displaced corals and abundant molluscs. Massive
