ELSEVIER
Marine Geology 127 (1995) 73-86
Pre-Sangamonian
eolianites in the Bahamas? New evidence from
Eleuthera Island
Pascal Kindler a, Paul J. Hearty b
aDPpartement de Gkologie et de Pakontologie, Universitt!de GenPve, 13, rue des Marafchers, 1211 GenPve 4, Switzerland
b Cable Beach Villas # 43, P. 0. Box N-3 723, Nassau, Bahamas
Received 20 March 1994; revision accepted 13 March 1995
Abstract
The present study provides new insight into the Quaternary stratigraphy of the Bahamas. Refining the stratigraphic
record of tectonically stable carbonate islands, such as the Bahamas, is important for improving our perception of
eustatic and climatic variations during this time interval.
Two key-sections located 15 km apart in the northern portion of Eleuthera display three vertically stacked rockunits: (1) a basal unit consisting of up to four eolianites separated by paleosols, (2) intermediate oolitic deposits
showing beach facies at +6 m, and (3) an upper bioclastic eolianite overlain by a thick calcrete.
The occurrence of beach facies at an elevation of 6 m above MSL suggests an Early Sangamonian age (isotopic
substage 5e) for the intermediate oolitic deposits. Because of its position below a thick calcrete, the upper unit was
more likely formed during a later highstand of the Sangamonian (substage 5c or 5a), rather than during the Holocene.
It follows that the basal unit clearly predates the last interglacial.
This new evidence from Eleuthera shows that pre-Sangamonian
rocks may be more extensive in the Bahamas
Archipelago than previously thought. It also emphasizes the widespread occurrence of bioclastic limestones in an area
often considered as being entirely composed of oolites.
1. Introduction
The study of Quaternary deposits exposed on
tectonically stable carbonate islands, such as
Bermuda or the Bahamas, can yield valuable information about past eustatic and climatic conditions.
Ancient sea-level elevations may be assessed from
sedimentological analysis, whereas examination of
diagenetic fabrics provides clues about paleoclimates. Setting these data in a coherent stratigraphic framework will clarify our perception
of eustatic and climatic changes during the
Quaternary and improve our understanding of
the processes controlling these changes. Refining
the stratigraphic record of the surficial deposits
found on these islands is thus very important.
OO25-3227/95/$9..50
0 1995Elsevier Science B.V. All rights reserved
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During the past 60 years, most geologists working on the isolated carbonate banks of the
Bahamas have focused their attention on modern
sediments and sedimentary environments (see references in Scholle et al., 1983) or on the internal
structure of the platforms (e.g. Mullins and Lynts,
1977; Sheridan et al., 1988). In contrast, stratigraphic investigation of surficial rock units has
only been conducted on a few islands (e.g. Garrett
and Gould, 1984; Carew and Mylroie, 1985).
Results from these earlier studies yielded the
following tenets of Bahamian geology: ( 1) the
Bahamas islands began to form by lateral accretion
of ridges around or between ancient headlands
during the last interglacial period (Garrett and
Gould, 1984); (2) pre-Sangamonian rocks have
14
P. Kind&
P. J. HeartyjMarine
been almost totally buried by these younger deposits and are now only visible in pits, caves or
quarries (Schlager and Ginsburg, 1981; Carew and
Mylroie, 1985); (3) present-day Bahamian topography essentially consists of oolitic eolianites
(Field, 193 1; Newell and Rigby, 1957; Beach and
Ginsburg, 1980; Schlager and Ginsburg, 1981).
This paper, as well as previous and ongoing
studies on several islands (Hearty and Kindler,
1993; Hearty and Kindler, 1995), demonstrates
that preSangamonian
units are more widespread
in the northwestern Bahamas than previously
thought. It further shows that bioclastic limestones
can locally form extensive ridges and that, in
certain areas, island growth occurred vertically
due to antecedent topography and high-energy
oceanic setting.
2. Setting and methods
Eleuthera is a long and narrow (100 x 2-5 km)
carbonate island, located on the northeastern and
windward margin of the Great Bahama Bank
(Fig. 1). It belongs to the tectonically passive
northwestern Bahamas (Sheridan et al., 1988) that
appear to be affected by slow (1.6 cm/lo3 years;
Lynts, 1970; Mullins and Lynts, 1977) subsidence
Geology 127 (1995) 73-86
largely due to thermally induced sedimentary loading (Pindell, 1985). Eleuthera lies close to the
platform edge and is fully exposed to the energetic
swells of the open Atlantic Ocean. Marine erosion
has cliffed most of the NE facing shoreline
and breached the island at one place (the Glass
Window, Fig. 2). Despite its accessibility, attractive cliffs and numerous roadcuts, few earth scientists have visited Eleuthera. Roehl (1967) and,
more recently, Foos (1991) undertook local studies
about soils and paleosols, but no comprehensive
stratigraphic research has ever been completed on
the island. In the following sections, we present
the results of recent investigations on two exceptional outcrops located in the northern part of
Eleuthera: at the Boiling Hole, near Gregory Town
(Figs. 1 and 2), and at The Cliffs, to the East of
James Point (Figs. 1 and 6).
Landform analysis, hand-sample petrography as
well as examination of sedimentary structures and
discontinuities were used to differentiate units
in the field. Twenty-five samples were collected,
impregnated with blue epoxy, thin-sectioned and
point-counted according to the method developed
by Chayes ( 1956) and revised by Fliigel ( 1982). A
minimum of two 200-grain counts were performed
on each thin-section to obtain the relative percentages of grains (ooids, peloids, bioclasts and
Fig. 1. Situation map of study area. Note location of Figs. 2 and 6.
P. Kindler, P.J. Hearty/Marine
&
k....:::.
L\ Glass
fi$;;;i>:.,
Window
Geology 127 (1995) 73-86
North A t/an tic
Ocean
Boiling Hole section
H
Holocene
sands
skeletal eolianite (5a)
***y
.$$j$j)l
............:...
..
:,=k
FI;I’lI politic marine deposits (se) “‘~:~~~~l., _ _
..
.,..,..
.......::
...:
..:::::::::::..
,.....
.,. ..*:
:.::j::::j:i’
. . .,...
,.,........
........
:~:::~:-4
i~~~~~
m
pre-Sangamonian eolianites
..::.:.:.:.:.:.:.~:.:.:.:...~
............
.:.I:
;::i,:iii:iiiii::,r:i
::y:,:,::.;:;:
500 m
:...:.::.:::
:..:.:..
,:.:.:.:.:.:
:.::...
I
..?I
:.::::::%::
Fig. 2. Geologic map of the Boiling Hole area.
miscellaneous), cements, primary and secondary
porosities (Table 1). Lastly, selected samples were
X-rayed in a Philips-Norelco XRG 3000 X-ray
diffractometer using Ni-filtered Cu radiation at the
University of Miami. Goniometer scans were conducted from 31’ to 25” (20) at a speed of lo per
minute to identify the main carbonate minerals.
3. Results
3.1. The Boiling Hole section
This outcrop (lat. 25”25’56”N, long. 76”35’54”W,
Fig. 2) is situated on the eastward, ocean-facing
shoreline of Eleuthera, about 800 m to the southeast of the Glass Window bridge. It makes a 75
m-wide re-entrant in a 4 km-long, 20 m-high seacliff and is best accessible during fair weather and
at low tide. The appellation “Boiling Hole” is not
reported on the official 1:25,000 topographic chart
of the area. It is, however, identified on a widely
distributed tourist-map and designates a natural
arch visible on the back wall of the outcrop. The
section is composed of three distinctive carbonate
rock bodies called hereafter Lower, Middle and
Upper Units.
Lower Unit
The Lower Unit (# 1, Fig. 3) forms a chain of
carbonate hillocks that have been cut obliquely by
erosional processes. The crests may reach elevations of up to 20 m above sea level, whereas
intervening depressions are locally submerged. The
upper surface of these limestones is capped by a
calcrete and breccia-rich clayey paleosol (Fig. 4a)
that has been stripped by marine erosion mainly
at low elevations (Fig. 4b). In addition to its wellpreserved dune-swale topography and orientation
parallel to the shoreline, the Lower Unit shows
many features recognized by McKee and Ward
(1983) as diagnostic of eolian deposition. These
P. Kindler, P. J. Hearty/Marine
16
Geology 127 (1995) 73-86
Table 1
Petrographic database used in this study
Sample
Setting
% Grains Pores
Cement
Biocl.
Ooids
Peloids
Misc.
Arag.
HMC
LMC
Hole section
Upper Unit
Boihg
EL 55
EL 69
0
0
2.8
11.6
2.3
8.5
13
16
11
0
1.6
4.5
1.1
6.8
3.1
4.3
15
65
68.8
59.1
34.6
44
66.9
18.8
23.6
13.4
29.4
18
35.1
18
6.2
9.8
0.1
9.2
40.6
11.8
10.8
60
0
40
IO
0
30
0
0
0
100
eolian
eolian
62.8
63
33.5
30.3
3.1
6.1
94.9
19.9
beach
beach
beach
beach
beach
beach
marine
61
65.3
66.5
60.5
68
64.3
68.4
11.3
2.8
0.1
8
2.5
4.5
3.4
21.1
31.9
32.5
31.5
29.5
31.2
28.2
Middle Unit
EL
EL
EL
EL
EL
EL
EL
58
51
56
54
53
52
66
Blocks at the base of the Middle Unit
EL 51
EL 70
? beach
? beach
68.5
65
2.3
4
29.2
31
4.9
0.4
31.9
86.5
39.6
6.8
11.6
6.3
eolian
eolian
eolian
eolian
58.8
54.6
51.4
58.5
13.3
6.1
13
18.5
21.9
39.3
29.6
28
16.1
6.6
8.2
2.8
5
8.2
15.6
23.4
62.8
11.2
51.4
68.2
16.1
14
24.8
5.6
beach
63.5
11.5
25
0.9
86.8
9.9
2.4
eolian
48
34
18
82.6
0
11.4
eolian
eolian
eolian
64
55
51.1
28
31.3
46
8
1.1
2.3
93.4
91.9
94
0
0
0
1
0.6
1.1
5.6
1.5
4.3
0
0
0
20
100
80
eolian
eolian
eolian
56.5
50.5
54.5
36
41
31.2
1.5
2.5
8.3
93
93.1
96.4
0
0
0
4.2
1.2
3.6
2.8
5.1
0
0
20
80
eolian
eolian
64
51.3
15.8
33.8
20.2
8.9
98.8
99.2
0
0
1.2
0.8
0
0
0
0
100
Lower Unit
EL
EL
EL
EL
65
64b
64a
61
The Cliffs section
Upper oolite
EL 83
Eolianite 4
EL 81
Eolianite 3
EL 24
EL 80
EL 19
Eolianite 2
EL 22
EL 78
EL 71
Eolianite 1
EL 21
EL 76
features include interstratified pedogenic horizons
and large-scale (up to 2 m) sets and cosets made
of steep (> 307, predominantly landward-dipping
foresets. Moreover, this rock body does not contain large marine shells such as commonly occur
in beach and subtidal carbonate sands and can
thus be identified as an eolianite. It is composed
of medium to coarse-grained, yellowish, peloidalskeletal limestone (Fig. 4b) showing a complex
pattern of equant and dog-tooth low-magnesium
calcite (LMC) cements indicative
diagenetic history.
of a diverse
Middle Unit
This second rock body (# 2, Fig. 3) is composed
of light-grey, horizontal beds that fill an interdunal
depression in the Lower Unit. The thickness of
this unit varies from zero on the sides of the
depression, to over six meters in the trough axis.
Pockets of coarse conglomerate containing calcrete
P. Kindler, P. J. HeartyjMarine
Geology 127 (I 995) 73-86
Fig. 3. View of the Boiling Hole section taken from the northwestern end of the outcrop, looking to the southeast. Middle oolitic
unit (2) fills a swale in the Lower Unit (I) and is overlain by the small hummocky dunes of the Upper Unit (3). White arrows
point to unit boundaries. Geologist to the far right is 1.8 m tall.
fragments and rounded blocks of oolitic-peloidal
laminated calcarenite locally occur at the base of
the unit. The grey horizontal beds consist of sparcemented oolitic-peloidal
calcarenites (Fig. 4e)
that present the following succession of sedimentary facies (Fig. 5): (a) a lower facies mostly
characterized by horizontal and low-angle crossstratification; (b) a intermediate facies showing
high-angle,
trough
crossmulti-directional,
bedding; (c) an upper massive interval of lowangle (< 10’) planar cross-beds (Fig. 4c) including
numerous irregular fenestrae near its top. This
upper interval is locally overlain by conglomeratic
layers and trough cross-bedded calcarenites displaying a distinctive polygonal pattern on their
upper surface (Fig. 4d). Trace fossils are abundant
within the two lower facies and include centimetersized, micrite-lined burrows attributed to the ichnogenus Ophiomorpha and short vertical shafts
perpendicular to bedding analogous to Skolithos
linear-is (Curran and White, 1991).
The basal conglomerate could correspond either
to a transgressive lag deposit or to a storm layer.
In the latter case, the rounded blocks of laminated
calcarenite could be fragments of beachrock trans-
ported seaward by powerful rip currents (Ward
and Brady, 1979); in the former, these blocks
would represent the remnants of an earlier transgression. This second hypothesis is supported by
the absence of diagnostic beachrock cements
within these blocks, as well as by the joint occurrence of calcrete fragments. The facies succession
observed in the horizontal beds can be interpreted
as a shallowing-upward
sequence from lower
shoreface to backshore
deposits. Low-angle
(< 10”) beach cross-beds occur at an elevation of
about 6 m above MSL. This succession of facies
is comparable to the sequences observed through
the Upper Pleistocene strandplains of northeast
Yucatan (Ward and Brady, 1979) and also
to deposits exposed at Clifton Pier on New
Providence Island. The conglomeratic layers and
trough cross-bedded calcarenites locally capping
the flat-bedded beach limestones probably represent storm berm and washover deposits respectively. The polygonal pattern visible on the upper
surface of these beds could be related to the
fracturing and collapse of a rapidly cemented
surficial crust or perhaps to shrinkage concomittant with cementation. The +6 m elevation of the
beach facies will be discussed in a later section.
78
P. Kindler, P. J. Hearty/Marine
Geology 127 (1995) 73-86
Fig. 4. Petrographic and sedimentologic features observed on the Boiling Hole section. (a) Calcrete (dark) and clast-rich clayey
paleosol capping the Lower Unit (f=rock fragments). (b) Sample EL 65. Discontinuity between Lower Unit (bottom) and Middle
Unit (top). Paleosol shown on previous photo has been stripped at this location. Black arrows point towards truncated peloids. (c)
Subhorizontal beach beds observed on the Middle Unit at the elevation of +6 m. Hammer handle is 20cm long. (d) Polygonal
pattern observed on the uppermost beds of the Middle Unit. See text for explanation. Hammer is 36 cm long. (e) Sample EL 52.
Spar-cemented oolitic-peloidal grainstone characterize the Middle Unit. Tangential ooids predominate in this microfacies, but ooids
with alternating tangential (white arrow) and radial (black arrow) layers also occur. (f) Sample EL 69. Poorly-cemented bioclastic
limestone including benthic forams (s=soritid) and red algae fragments (r) identify the Upper Unit. Note well preserved primary
porosity (p).
P. Kindler, P.J. Hearty/Marine
UPPER
UNIT
Geology 127 (1995) 73-86
19
The bioclastic composition of these limestones as
well as the presence of a thin intervening paleosol
clearly show that this upper eolianite does not
belong to the underlying oolitic-peloidal sequence.
3.2. The Cl@s section
MIDDLE
UNIT
Skolithos
LOWER
UNIT
LEGEND
a
rhizoliths
m
fenestrae
calcrete
txi
I.
L++J herring-bone
Ed eolian
El beach
El trough
str.
EL65
foresets
bedding
cross-bedding
sample numbers
Fig. 5. Sedimentologic log of the Boiling Hole section.
Upper Unit
Small hummocky dunes characterize the Upper
Unit (# 3, Fig. 3) which overlies both rock bodies
previously described. Up to 3 m thick, this unit is
capped by a micritic crust and underlies unconsolidated modern sand that is not considered in this
study. Overall morphology and the presence of
steep (> 30”), landward-dipping foresets suggest
that sedimentation of these deposits took place in
an eolian environment. Pervasive root casts, the
absence of large marine shells and the common
occurrence of early fresh-water vadose cements
(Fig. 4f) also support such a setting (McKee and
Ward, 1983). The Upper Unit is composed of welllithified, coarse-grained skeletal grainstones containing benthic foraminifera, mollusk debris and
fragments of coral and calcareous algae (Fig. 4f ).
X-ray diffraction (XRD) analysis reveals that most
of these skeletal fragments have retained their
original mineralogy (aragonite or high-Mg calcite).
This outcrop (25”20’1 l”N, 76”24’19”W, Fig. 6)
corresponds to a re-entrant at the eastern end
of the 10 m-high, 3 km-long rocky cliff section
stretching to the East of James Point. This location
is not named on the official topographic chart of
Eleuthera, but is also identified on the tourist-map
mentioned earlier.
Freshly exposed by shoreline retreat, The Cliffs
section displays an impressive vertical succession
that includes four eolian rock bodies bounded by
reddish pedogenic horizons (Figs. 7, 8a and 9).
This sequence is covered inland by oolites recently
exhumed during Hurricane Andrew and by unconsolidated washover and dunal sand of Holocene
age (Figs. 6 and 7). The eolianites, labelled 1
trough 4, were identified on the basis of their
overall morphology and large-scale sedimentary
structures which include thick (> 2 m) sets of steep
(> 30”), predominantly landward-dipping, convexupward foresets (eolianite #l, Fig. 8a). Rare lowamplitude wind-ripples (ripple index > 25, Fig. 8b)
and pervasive rhizocretions, especially in the upper
two units (#3 and #4, Fig. 8d), further support the
eolian nature of these deposits. The paleosol capping eolianite #2 corresponds to a well-developed
karstic surface. It displays a thick micritic crust,
black pebbles and breccia pockets (Fig. 8c) and
further yielded tall, finely-ribbed land snails (Cerion
sp.). The paleosol overlying the uppermost
unit also represents an important discontinuity,
whereas those covering eolianite #l and #3 are less
prominent and only show patchy ferruginous
crusts and mottled zones.
The petrographic composition of the eolianites
exposed at The Cliffs is constant throughout the
section (Table 1). All units are made of medium
to coarse-grained, well-lithified bioclastic limestone
containing numerous abraded benthic foraminifers
(soritids, miliolids, rotaliids) and abundant coral
and algal debris (Fig. 8e). Mollusk fragments also
P. Kindler, P. J. HeartyJMarine
80
I
The Cliffs section
NorN7Atlantic Ocean
5OOm
m
Holocene
skeletal
Geology 127 (1995) 73-86
D
Little Bluff
sands
a
eolianite
(5a) m
oolitic marine deposits (se)
pre-Sangamonian
eolianites
Fig. 6. Geologic map of The Cliffs area. Prior to Hurricane Andrew (August 1992), the limit between Early Sangamonian oolites
and older skeletal eolianites was totally covered by Holocene sands.
Fig. 7. View of the western promontory of The Cliffs section (from the West). Pre-Sangamonian eolianites (#I-4) are overlain by
recent dunal and washover sands (h). Sangamonian oolitic ridge (0) is visible in the background. Large white erosional scar was
made by Hurricane Andrew. Thrown-over blocks (left) give an idea of storm strength. Cliff height is 12 m.
occur but they are commonly leached and replaced
by sparry calcite. Allochems are bound by LMC
equant spar located at grain contacts and locally
filling intergranular pores. In a few samples, this
calcitic cement is overlain by fibers of high-Mg
calcite (HMC) and/or internal silty sediment prob-
ably originating from marine spray (Fig. 8f). XRD
analysis shows that The Cliffs limestones are essentially made of low-Mg calcite. This result along
with other petrographic data (e.g. micrite envelopes) shows that these rocks have been pervasively
altered by meteoric waters.
P. Kindler, P.J. Hearty/Marine
Geology 127 (1995) 73-86
81
Fig. 8. Petrographic and sedimentary features observed on The Cliffs section. (a) View of the western wall of the outcrop showing
vertical stacking of four eolian rock bodies (#I-4) separated by paleosols. Note steep (32”), landward-dipping foresets in eolianite
#l. Geologist standing on cliff edge is 1.7 m tall. (b) Well-preserved, low-amplitude wind-ripples found on lowermost eolianite.
Hammer is 36 cm long, (c) Detail of calcrete and breccia-rich paleosol capping eolianite #2. Arrow points to black pebble. Upper
part of the photo represents the base of eolianite #3. Hammer for scale. (d) Pervasive rhizoliths within uppermost eolianite. Arrow
points to hammer handle. (e) Sample EL 21, eolianite #l. Bioclastic limestone (s=soritid, r= red algae fragment). White arrow
points to non-isopachous fringe of equant spar cement (“grain-skin” cement, Land et al., 1967). Black arrow points to pedogenic
micritic rind. (f) Sample EL 81, eolianite #4. Bioclastic limestone showing three generations of cements: equant spar (black arrow
to the left), syntaxial overgrowth (s) around an echinoid fragment and isopachous fringes of aragonite (white arrow) precipitated
from sea spray.
P. Kindler, P. J. Hearty/Marine
82
Cerion
Geology 127 (1995) 73-86
units remains hazardous. Refined uranium-series
dating may yield precise ages for Upper Pleistocene
formations (e.g. Chen et al., 1991) as long as the
analyzed material consists of perfectly preserved
coral. In a previous paper (Hearty and Kindler,
1993) we measured amino-acid ratios on wholerock samples to obtain some information about
the relative age of the lithostratigraphic
units
exposed on San Salvador Island. This technique
has recently been contested (Carew and Mylroie,
1994). We maintain that amino-acid racemization
is a useful method to resolve the chronosequence
(Hearty and Kindler, 1994). However, we want to
demonstrate, in this paper, that the use of classic
morphostratigraphic
and sedimentologic
techniques are adequate to unravel the stratigraphy of
the Bahamas islands.
4.1. Age of Boiling Hole units
LEGEND
1,k ,l.
m
EL 76
rhizoliths
paleosol
ea
eolian foresets
ta
eolian topsets
sample numbers
Fig. 9. Sedimentologic log of The Cliffs section. Eolianite #I
and #2 could correspond to the Owl’s Hole Formation (stage
9, Hearty and Kindler, 1993). Eolianite #3 and #4 may be
equivalent to the Fortune Hill Formation (stage 7, Hearty and
Kindler, 1993).
4. Discussion
The difficulty of dating Quaternary carbonates
in part explains why our knowledge of Bahamian
stratigraphy is still scanty. Indeed, only one group
of animals, the land snail Cerion, evolved rapidly
enough to provide some degree of paleontological
resolution for these deposits (Garrett and Gould,
1984). Radiocarbon dating on whole-rock samples
is a helpful method for identifying Holocene deposits (e.g. Kindler, 1992), but its application to older
Middle Unit
In tectonically stable regions, such as the
Bahamas, perched strandline features provide
valuable information about past sea-level elevations. At the Boiling Hole, the position of beach
facies at + 6 m gives a good approximation of sealevel stand upon deposition of the middle oolitic
unit. Global (e.g. Chappell and Shackleton, 1986;
Selivanov, 1992) and regional studies (Neumann
and Moore, 1975; Chen et al., 1991) established
that, in Late Quaternary times, eustatic highs well
above modern datum only occurred at the beginning of the last interglacial, between 140,000 and
120,000 yrs B.P. (Fig. 10). A Holocene highstand
of similar amplitude has been proposed by White
and Curran (1993) to account for the local occurrence of sedimentary structures typical of a backshore setting in contemporary strata located at + 7
m on Lee Stocking Island (Exumas). However,
the relationship of their sedimentological data to
present sea-level can also be explained in a different
way (Kindler,
1995). These authors further
acknowledge that there is no published evidence
of such a recent highstand from adjacent areas in
the Bahamas. In any case, the presence of a
stratigraphic succession including two paleosols
and a well-lithified eolianite above the Middle Unit
suggests a Pleistocene rather than a Holocene age
P. Kindler, P.J. Hearty/Marine
BH
2.2
0
MIKE
U&
UNIT
l&2
3&4
>
83
Geology 127 (1995) 73-86
LCIKR
UNIT
I
I
I
I
100
200
300
400
ka BP
Fig. 10. The rock-units described in this paper are placed on a @*O curve for the past 400 kyr (Imbrie et al., 1984), a proxy record
of sea-level high and lowstands for this time interval. BH stands for Boiling Hole, TC for The Cliffs.
They presumably date from the
Early Sangamonian (oxygen-isotope substage 5e)
and are coeval with the Grotto Beach Formation
on San Salvador (Carew and Mylroie, 1985;
Hearty and Kindler, 1993). The discovery of
140,000 year-old coral fragments within a shallowing-upward sequence of oolitic-peloidal deposits,
similar to the Middle Unit, at Clifton Pier (New
Providence Island; Neumann and Moore, 1975)
also suggests this age.
for these deposits.
Upper Unit
Its position above the oolitic limestones composing the Middle Unit and the occurrence of an
intervening paleosol show that the upper skeletal
eolianite is younger than isotopic substage Se.
The presence of a capping calcrete and reddish
pedogenic horizons, the abundance of root casts
and the degree of induration, all support a
Late Sangamonian rather than a Holocene age
( Kindler, 1995). This unit may have been deposited during oxygen-isotope substage 5c (100,000
yrs B.P.) or 5a (85,000 yrs B.P.) when sea level
was apparently high enough to allow eolian deposition on the bank margin (Fig. 10). It could thus
correspond to the Almgreen Cay Formation
recently defined on San Salvador (Hearty and
Kindler, 1993) and to the Southampton Formation
in Bermuda (Vacher and Hearty, 1989).
Lower Unit
Its basal position and the pronounced discontinuity at its top indicate that the lower eolianite,
which is the largest exposed unit at the Boiling
Hole (Fig. 2), predates the Sangamonian interglacial. We do not have further precision regarding
the exact age of these deposits. However, they
show morphologic and petrographic characteristics similar to the Hunt’s Cave Formation sediments on New Providence Island, which yielded
amino-acid ratios coherent with an age ranging
from 300 to 400 kyr B.P. (stage 9 or 11, Hearty
and Kindler, 1995; Fig. 10).
The Boiling Hole section thus presents a perfectly exposed vertical succession of three distinctive Pleistocene rock bodies that has up to now
never been described on any Bahamian island.
This sequence includes a basal unit older than
the Sangamonian, an intermediate unit of Early
Sangamonian age, and an upper unit younger than
substage 5e, but predating the Holocene. It is
important to stress the wide lateral extension of
the pre-Sangamonian unit which can be followed
northwestward to the Glass Window bridge and
beyond over a distance exceeding 2 km.
84
P. Kindler, P. J. Hearty/Marine
4.2. Correlation of the Boiling Hole and The Clifss
sections
Detailed morphologic, petrographic and sedimentological examination of both outcrops shows
that the oolite recently exhumed by Hurricane
Andrew at The Cliffs can be correlated with the
Middle Unit at the Boiling Hole, and must likewise
date from isotopic substage 5e. The four eolian
rock-bodies composing the cliff face at The Cliffs
(Figs. 7, 8a and 9) thus predate the Sangamonian,
but might not necessarily correspond to the Lower
Unit at the Boiling Hole, due to distinctive petrographic characteristics (Table 1). The pronounced
discontinuity surface that separates eolianites #l
and #2 from eolianites #3 and #4 clearly represents
a considerable amount of time as shown by associated breccia layers, black pebbles and thick micritic
crusts (Fig. 8~). This surface is similar to that
capping eolianite #4 and, likewise, represents a
major sedimentation break during a glacial period.
Deposits from two distinct interglacial events predating the Sangamonian, possibly isotopic stages
7 and 9, are thus exposed at The Cliffs. This new
data confirms the occurrence of two MiddlePleistocene bioclastic limestone units in the
Bahamas islands identified by Hearty and Kindler
(1993) on the basis of amino-acid ratios.
The weathered skeletal eolianite overlying the
Lower Sangamonian (5e) oolites in the vicinity of
The Cliffs outcrop (Fig. 6) shows marked petrographic, morphologic and sedimentological resemblances with the Upper Unit exposed at the Boiling
Hole. It can therefore be considered as a Late
Sangamonian (substage 5a or 5c) depositional
event and further emphasizes the similarity
between both exposures.
4.3. Modes of island accretion
A first glimpse at both studied areas reveals that
carbonate islands, such as the Bahamas, may also
grow by vertical stacking of deposits as well as
laterally as it is commonly believed (Vacher, 1973;
Garrett and Gould, 1984). At the Boiling Hole,
Lower Sangamonian marine oolites accumulated
in a depression formed within older deposits. At
The Cliffs the same facies result from trans-island
Geology 127 (1995) 73-86
circulation behind a Middle Pleistocene ridge.
Antecedent topography thus appears as a prominent factor controlling island growth. The proximity of the studied outcrops to the bank margin
suggests that vertical stacking of stratigraphic units
in this region may also be related to the narrowness
of the outer platform and exposure to a highenergy oceanic environment.
4.4. Petrographic composition of Pleistocene units
The present study emphasizes the wide occurrence of bioclastic eolianites of Pleistocene age
along the Eleuthera shoreline, particularly in The
Cliffs sector (Table 1). Oolitic dunes are also found
in this area (Bain and Kindler, 1994), but they are
not predominant. Hearty and Kindler (1993) made
similar observations on San Salvador. At least
three phases of bioclastic sedimentation occurred
during Pleistocene times. The two oldest phases
clearly predate the Sangamonian
interglacial,
whereas the youngest phase took place towards
the end of this time interval. It is thus inaccurate
to consider the Bahamian islands as mostly
composed of oolitic deposits (Newell and Rigby,
1957; Beach and Ginsburg, 1980; Schlager and
Ginsburg, 198 1) .
5. Conclusions
Our detailed study of the geomorphic, sedimentologic and petrologic characteristics of the
deposits exposed at the Boiling Hole and at The
Cliffs in northern Eleuthera demonstrates that preSangamonian rock units are predominant in this
portion of the island and may be more widespread
in the Bahamas than previously thought. This
study also shows that bioclastic eolianites are
locally extensive, thus modifying the common
opinion that all Bahamian ridges are oolitic.
Lastly, our understanding of carbonate platform
evolution is refined by recognition that carbonate
islands, such as the Bahamas, may grow vertically
as well as laterally, depending upon antecedent
topography and energy setting. Better constraining
the factors of island growth is important for these
islands have a profound effect on the distribution
P. Kindler, P.J. HeartyJMarine Geology 127 (1995) 73-86
patterns of subsequent
platforms.
sediment
on carbonate
Acknowledgements
This study was undertaken during Kindler’s
tenure at the University of Miami, Fla. and was
supported by the National Science Foundation
of Switzerland (grant # 8220-028458/l, projects
# 20-29917.90 and 20-40638.94). G. Gorin,
A. Strasser, D. Steffen, M. Joachimsky and A.
Haldimann-Kindler have been very helpful collaborators in the field. Reviews by A.C. Hine, A.C.
Neumann and three unknown colleagues greatly
contributed to the improvement of the original
manuscript. Special thanks to Dr. G. Brass (Univ.
Miami) for free use of X-ray diffractometer and
to the Cambridge family in Gregory Town,
Eleuthera, for their friendly support.
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