5th Swiss Geoscience Meeting, Geneva 2007
Paleosol versus colluvium: records of sea-level
fluctuations versus catastrophic events in the
Quaternary of The Bahamas
Godefroid Fabienne*, Brentini Maud*, Kindler Pascal*, Nawratil de Bono
Carole* & Samankassou Elias**
*Department of Geology-Paleontology, University of Geneva, 13 rue des Maraîchers,
CH-1205 Geneva (fabienne.godefroid@terre.unige.ch)
**Department of Geosciences, University of Fribourg, Pérolles, CH-1700 Fribourg
The adequate recognition of pedogenic layers is an important key in Bahamian
geology, as well as in sea-level and climate reconstruction globally. For an
unambiguous identification of paleosols, a distinction between pedogenic and
colluvial phenomena is required. Furthermore, this discrimination can allow
identifying major catastrophic events in sedimentation that are independent
from sea-level fluctuations such as tsunamis and landslides.
Here we report on one example from Hatchet Bay, located on Eleuthera Island,
The Bahamas, between Surfer’s Beach and Rainbow Cay. This outcrop is
important because it displays a stratigraphic sequence that includes both
paleosol and colluvium horizons (Nawratil de Bono 2005).
The Hatchet Bay outcrop exposes three units (fig.1):
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Unit 1 comprises oolitic and bioclastic strata of Pleistocene age with lowangle planar bedding and fenestrae. The uppermost bed is bioturbated
and overlain by an orange crust. This unit is interpreted as a beach
deposit.
Unit 2 is a 30-cm thick conglomeratic layer with a red clayey matrix and
decimetric to metric angular boulders. This red layer contains also black
pebbles and marine shell fragments. The boundary between the beach
and conglomerate layers is erosive.
Unit 3 is an unlithified, bioclastic Holocene ridge affected by marine
erosion. Foreset structures are not visible, but this unit can be identified
as an eolianite.
As observed by Nawratil de Bono et al. (2001), Unit 2 is conspicuous. It lacks
pedologic horizons although the clayey-sandy matrix may look similar to that of
a paleosol. In-situ roots are absent and blocks are chaotically arranged in the
matrix. Therefore, this layer must be interpreted as a colluvium rather than a
paleosol.
Colluvium is related to a sudden phenomenon, such a slide that leads to the
accumulation of rock debris at the foot of steep slopes. This deposit is the result
of runoff due to rainfall or of violent, catastrophic events such washovers or
tsunamis. Colluvia are not linked to sea level and may thus occur during
highstands and lowstands. In contrast, paleosols are the result of pedogenic
processes that act on a substrate. The formation of a paleosol lasts very long
5th Swiss Geoscience Meeting, Geneva 2007
(up to 100’000 years). The colonisation of a carbonate substrate by vegetation
takes long and requires favourable climatic conditions.
This example shows the importance of discriminating between pedogenic and
colluvial events, allowing to distinguish eustatic variations from catastrophic
events in the reconstruction of sea-level fluctuations and the interpretation of
climate changes. Furthermore, this case study demonstrates that red-coloured
horizons need to be studied and interpreted with care because the latter may be
linked to pedogenic or colluvial events, each recording a distinct phenomenon.
Figure 1. The Hatchet Bay outcrop. Person for scale is 182 cm.
REFERENCES
Nawratil de Bono, C. 2005: Pétrographie, micromorphologie et minéralogie des
paléosols pléistocènes d’Eleuthera, Bahamas : Enseignements
paléoenvironnementaux. PhD Thesis, University of Geneva, 284 pp.
Nawratil de Bono, C., Kindler, P. & Cojan, I. 2001: The Terra Rossa paleosol
from Hatchet Bay, Eleuthera, Bahamas: a real paleosol? Abstracts, Ninth
Meeting of Swiss Sedimentologists, Fribourg, 30.
This research is sponsored by the Swiss National Research Funds (project n°200020-113356).