Geoscience Reference
In-Depth Information
Porites species and head corals of Montastraea canalis are clearly not in situ, as they
show different orientations with respect to bedding. The sandstone beds become thin-
ner and fi ner upsection, whereas the interbedded siltstone beds thicken. In addition, the
sandstone beds grade upsection from a medium-grained calcarenite to a fi ne-grained,
lithic wacke, such that carbonate grains decrease in abundance, whereas quartz and
lithic grains increase in abundance. There is also a lateral facies change between these
two facies, such that the sandstone beds become thinner and the siltstone beds thicken
to the north over 1 km of exposure. The siltstone beds contain abundant foraminifera,
molluscs, echinoids, shark teeth, and burrows. A distinctive white ash bed (4 cm thick)
is present at the base of a tuffaceous sandstone bed (1 m thick). This ash bed and over-
lying tuffaceous sandstone, which serves as a useful marker bed (Figure 6), is overlain
by a thick interval of siltstone (~30 m). Overlying this thick interval of siltstone is
the uppermost interval of the upper member, which consists of alternating sandstone
and mudstone beds with local conglomerate and lignite beds (Sections 3-5, 7, ECB-
3, ECB-5, and GH-10). The sandstone beds become thicker and coarser upsection,
whereas the mudstone beds become thinner and more carbonaceous upsection. A dis-
tinctive lignite bed is present in drill holes ECB-3 and ECB-5, which serves as a use-
ful marker bed (Figure 6). Carbonaceous mudstone beds commonly contain horizons
of carbonized to permineralized wood of mangrove trees. Branches and prop roots
are commonly encrusted with oysters (
Crassostrea aff
. C. virginica) and are com-
monly bioeroded by teredinid bivalves (Kuphus “incrassatus”). Also common in the
carbonaceous mudstone are poorly preserved seeds, gastropods (
Turritella venezuelana
,
Turritella
(Bactrospira?)
amaras
,
Potamides suprasulcatus
), bivalves and crustaceans.
The top of the Culebra Formation is defi ned by the incision into either carbonaceous
mudstone or sandstone (depending upon location) of a channel infi lled with pebble
conglomerate containing wood without teredinid borings of the Cucaracha Formation.
The lower half of the upper member represents continuing transgression through
time; whereas, the upper half represents the start of a regression with shallowing water
depth through time (Figure 11). The lowermost calcarenite beds above the E.L. rep-
resent open-shelf, neritic conditions. The decrease in bed thickness, in grain size and
in carbonate content of these sandstone beds, as well as the thickening siltstone beds
upsection, indicate increasing water depth through time. This interpretation is consis-
tent with Blacut and Kleinpell (1969), who described benthic foraminifera represent-
ing upper bathyal depths (~200-400 m) from these siltstone beds. This interpretation
is also consistent with Van den Bold (1972), who described ostracodes representing
“deep-water” depths. During deposition of the middle portion of the upper member,
water depth began to shallow, with sandstone beds increasing in frequency, thickening
and coarsening, and mudstone beds becoming thinner and increasing in carbonaceous
plant content. All of these observations suggest a small prograding, river-dominated
delta (Bhattacharya and Walker, 1992; Pettijohn et al., 1987), perhaps on a similar
scale to the present-day Rio Grande in Bocas del Toro, Panama. The thick siltstone in-
terval represents the prodelta, whereas the overlying sequence of alternating mudstone
and sandstone beds represent the delta front (Figure 11). Sandstone interbeds represent
distal distributary mouth bars. Local lenses of pebble conglomerate represent
