Cent. Eur. J. Geosci. • 3(1) • 2011 • 71-76
DOI: 10.2478/s13533-011-0009-3
Central European Journal of Geosciences
Pleistocene trace fossils in the Ironshore Formation,
Little Cayman, British West Indies
Communication
Robert Metz∗
Department of Geology and Meteorology,
Kean University, Union 07083, New Jersey, USA,
Received 31 January 2011; accepted 19 February 2011
Abstract: Oosparite grainstones of the Pleistocene Ironshore Formation, Little Cayman, British West Indies, contain
the trace fossils Conichnus conicus, Ophiomorpha nodosa, and Planolites beverleyensis. The dominance of
vertically-oriented trace fossils, complexity of cross-stratifications, coarseness of the sediment channel fill, and
presence of several rudstone layers suggest deposition close to the seaward portion of lagoonal channels where
higher energy conditions prevailed.
Keywords: trace fossils • Ironshore Formation • Pleistocene • Little Cayman • carbonates
© Versita Sp. z o.o.
1.
Introduction
Much has been stated about the need for additional trace
fossil research on carbonates, as well as the associated
problems (e.g., diagenetic alterations) that may be encountered [1–3]. Nonetheless, several significant studies
have already been reported, particularly from the Paleozoic (e.g., [4–7]), as well as the Pleistocene (e.g., [2, 8–
12]). In addition, several authors have made a strong case
for considering certain shallow water carbonates and their
enclosed trace fossils environmentally comparable to their
siliciclastic equivalents [2, 6, 11].
The Cayman Islands include three small islands, Grand
Cayman, Cayman Brac, and Little Cayman. Interestingly, although detailed geologic as well as trace fossil
research has been conducted extensively on Grand Cay∗
E-mail: [email protected]
man (e.g., [2, 13–17]), limited geologic (e.g., [18]) and no
ichnologic studies have occurred on Little Cayman. All
three islands are described as having a core consisting
of Oligocene-Miocene carbonates (Bluff Formation), surrounded and partly onlapped by limestones comprising
the Pleistocene Ironshore Formation [16]. [17] divided the
Ironshore Formation on Grand Cayman into four (A-D) unconformable units (recently, [19] detailed additional information from Grand Cayman on another two, younger units
labeled E and F). Using Th/U dates from corals [17] determined that the Ironshore Formation on Grand Cayman
ranged in age from >400,000 to approximately 131,000
years (A-D, older to younger, respectfully). They noted
that the carbonate sediments of the Ironshore Formation
accumulated ∼125,000 years ago (e.g., [20]) and formed
under largely lagoonal conditions, during interglacial,
sea-level highstand, with waters approximately 6 m above
present-day sea level (e.g., [21]).
An initial reconnaissance of Little Cayman indicated that
Salt Rocks, located on the northwestern portion of the
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Pleistocene trace fossils in the Ironshore Formation, Little Cayman, British West Indies
island, provided the best exposure of the Ironshore Formation as well as trace fossil representation (Fig. 1). [18]
provided the only detail from this section. He noted that
the Ironshore Formation consisted of a lower reef facies
overlain by oolitic calcarenites (= oosparite grainstones)
comprising the sand ridge facies. The sand ridge facies
(2 m thick, approximately 550 m in lateral extent) of [18]
includes three components: a lower coarse grainstone
containing rounded, oolitic blocks and Strombus shells;
a seaward dipping bedded calcarenite (= oosparite grainstone); and an overlying gently dipping calcarenite (=
oosparite grainstone). [18] also noted that similar oolitic
calcarenites overlay a reef facies complex on Grand Cayman.
ranges from 1.5 to 10 cm, width at base 1.2 to 4 cm; maximum length observed 12 cm.
Remarks
Conichnus is considered to represent the dwelling and/or
resting burrow of an anemone-like organism (e.g., [22–24]).
Seven specimens recorded in the field.
Ichnogenus Ophiomorpha Lundgren, 1891
Ophiomorpha nodosa Lundgren, 1891
Figure 2C-D
Diagnosis
Burrow walls consisting of dense, regularly distributed
discoid, ovoid, or irregular polygonal pellets (after [25]).
Description
Horizontal to vertical, sporadically Y-branched burrow
systems, up to 15 cm in length, preserved in full relief.
Burrows 1-4 cm in diameter, have well-developed walls up
to 3 mm thick. Exterior of burrows consists of individual,
mammilated ovoid pellets, 3-4 mm in diameter; interior
walls are smooth. Many burrow segments are unfilled,
those filled consist of sediment similar to the surrounding
matrix.
Remarks
Figure 1.
Location map.
The main purpose of this note is: 1) to document trace
fossils from the middle unit of the sand ridge facies comprising the upper portion of the Pleistocene Ironshore Formation, Salt Rocks, Little Cayman, and 2) to discuss the
likely subenvironments.
2.
Systematic ichnology
All specimens were recorded in the field.
Ichnogenus Conichnus Männil, 1966
Conichnus conicus Männil, 1966
Figure 2A-B
Diagnosis
Indistinctly to thinly lined vertical conical burrows which
taper to a smooth, rounded, but distinct basal apex (after [2]).
Description
Largely vertical to slightly subvertical, conical burrows,
exhibiting V-shaped to U-shaped (in oblique section), convex downward, nested internal laminations. Width at top
Ophiomorpha is commonly interpreted as a marine
dwelling structure formed by a suspension-feeding,
shrimp-like organism (e.g., [26]), although nonmarine examples have been reported [27]. In particular, Ophiomorpha nodosa is commonly associated with higher energy
marginal marine (e.g., shoreface) environments [26]. A
strong case can be made for callianassid shrimps as
tracemakers responsible for Little Cayman Ophiomorpha
(see [12]). Interestingly, exposure of bedding plane surfaces bearing trace fossils is relatively rare in the Pleistocene Ironshore Formation [2], although a few such exposures do occur at Salt Rocks, Little Cayman (Fig. 2C-D).
At least 150 specimens recorded in the field.
Ichnogenus Planolites Nicholson, 1873
Planolites beverleyensis (Billings, 1862)
Figure 2E
Diagnosis
Relatively large, smooth, straight to gently curved or undulose cylindrical burrows (after [28]).
Description Mostly straight to gently curved, horizontal,
smooth, unbranched, cylindrical, unlined burrows. Burrow
diameter 4-5 mm, maximum observed length 80 mm. Burrows parallel to bedding; fillings structureless and differ
from surrounding rock, being somewhat more uniform in
texture.
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R. Metz
Figure 2.
Ichnofossils from the Ironshore Formation at Salt Rocks. A-B. Conichnus conicus. Scale for A = 5 cm. C-D. Ophiomorpha nodosa.
Scale for D = 5 cm. E. Planolites beverleyensis.
Remarks
Planolites likely represents the active backfilling of a burrow by a mobile deposit-feeder [28]. It is a facies-crossing
ichnofossil, occurring in a variety of marine (e.g., [29]) and
nonmarine environments (e.g., [30]). Though close in form
to Macaronichnus in having a modified fill, Planolites differs from Macaronichnus in lacking a lined, often darkgray mantle. Ten specimens recorded in the field from
a single bedding surface.
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Pleistocene trace fossils in the Ironshore Formation, Little Cayman, British West Indies
3.
Discussion
At Salt Rocks, Little Cayman, the bedded sand ridge
facies (2 m thick) of [18] comprising the upper portion
of the Ironshore Formation, is exposed several hundred
meters laterally. Of particular interest are the seaward dipping oosparite grainstones (middle unit of [18])
characterized by a diversity of ichnotaxa and physical
sedimentary structures. The ichnofossils, dominated by
Ophiomorpha nodosa, also include Conichnus conicus,
and Planolites beverleyensis. Sedimentary structures
include high-angle cross-stratifications, multidirectional
high-angle trough cross-stratifications and coarser, sporadically graded channel fills (Fig. 3A-C). Furthermore,
rudstone formed of lithoclasts (up to 0.5 m in length) of
oosparite grainstone is found at several horizons within
the middle component of the sand ridge facies (Fig. 3D).
The ichnological assemblage with its dominantly
vertically-oriented burrows of dwelling organisms, sedimentary structures, and sporadic rudstone layers, suggest a shallow marine environment subject to increasing energy conditions. As such, higher energy waters likely filled previously eroded channels with sandto pebble-size, sporadically graded particles. Interestingly, the only occurrences of Planolites beverleyensis are associated with dominantly horizontal burrows
of Ophiomorpha nodosa on a single bedding surface
of oosparite grainstone. [31] noted that under siliciclastic settings, largely horizontally-oriented burrows of
Ophiomorpha suggest lower energy environments while
largely vertically-oriented ones suggest higher energy
conditions. Thus, this may represent a temporary energy
lull during deposition of this component of the Ironshore
Formation. There is also evidence of sporadic storm activity in the form of several rudstone layers.
[2] and [32] reported on seven ichnotaxa, including ones in
the present discussion, from the upper portion of the Ironshore Formation on Grand Cayman. Of particular interest are the ichnotaxa-bearing, high-angle cross-stratified
oolitic limestones and their associated lithoclasts, which
were interpreted to have been deposited in channels of
a lagoon, under shallowing-upward heightened energy
conditions (see [32], for details). [11] noted the potential significance of one of these mutually occurring ichnofossils (Conichnus conicus), as an indicator of a lagoonal
or lagoonal subenvironment. Investigation at a quarry,
approximately 5.5 km southeast of Salt Rocks (Fig. 1),
revealed molluscan-enriched (largely bivalves), somewhat
massive, friable packstone, interpreted to be Ironshore
strata of lagoonal origin. It is quite plausible that the
shallowing-upward conditions at Grand Cayman [32] and
the Bahamas [11], generated by glacio-eustatic sea-level
Figure 3.
Ironshore Formation at Salt Rocks, Little Cayman. A-B.
High-angle cross-stratification and multidirectional highangle trough cross-stratification. Note Ophiomorpha to
right in A. Scale = 5 cm. C. Graded channel fill. Scale
= 15 cm. D. Rudstone lithoclasts of oosparite grainstone.
Hammer length = 27 cm.
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R. Metz
change also affected Little Cayman during deposition of
the upper portion of the Ironshore Formation. Thus, at
Salt Rocks this could have resulted in filling of the lagoon, whose sediments prograded over the earlier formed
reef facies of the Ironshore Formation. Subsequently, sporadic higher energy conditions resulted in channel scouring and later filling. At Salt Rocks, the complexity of the
cross-stratifications, dominantly vertically-oriented trace
fossils, as well as the coarseness of the sediment channel
fill suggests deposition closer to the seaward portion of
the channels where higher energy conditions likely prevailed.
A point to consider is the degree of diversity of the trace
fossils found at Little Cayman versus Grand Cayman [2].
Why are there only three ichnogenera on Little Cayman
versus seven on Grand Cayman? The ichnofossil-rich section near Salt Creek on Grand Cayman varies from 3-4 m
thick [32], while the actual ichnofossil-rich middle portion
of the sand ridge facies at Salt Creek on Little Cayman is
approximately 1 m thick. As such, since at both locations
the Ironshore Formation is largely confined to vertical exposures, the greater thickness at Grand Cayman could
account for the diversity difference between the two areas. This does not mean that at one time a comparable
thickness as well as higher diversity of trace fossils may
not have been present on Little Cayman. However, Little
Cayman, having the least topography of all the Cayman Islands may have subject to much greater periods of erosion,
and as such lost significant portions of this ichnofossil-rich
layer.
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
Acknowledgments
I thank H. Allen Curran for reviewing an earlier draft of
this paper. I also thank the two anonymous reviewers
for their useful comments and suggestions. I thank Carrie Manfrino, President of the Central Caribbean Marine
Institute, for suggesting and supporting this research on
Little Cayman.
[12]
[13]
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