CIG   05423
CENTRO DE INVESTIGACIONES GEOLOGICAS
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Estimating aggradation rates from palaeosol type: the Punta San Andrés Alloformation (Argentina) as a case study.
Autor/es:
BEILINSON, ELISA
Lugar:
Bochum, Alemania
Reunión:
Congreso; 26th IAS Meeting of Sedimentology; 2008
Institución organizadora:
International Association of Sedimentology y Ruhr University-Bochum
Resumen:
The degree of soil development
depends on how long sediment is exposed to pedogenic processes (Marriott and
Wright, 1993). That is to say that, whenever sedimentation rate is low, palaeosols
will develop more mature profiles than those developed under higher sedimentation
rates. Also, certain pedogenic features are very indicative of sedimentation
rates. Hydromorphic and vertic features such as nodules, slickensides and
cutans develop under relatively high aggradation rates while the development of mature calcrete profiles
require lower aggradation rates (Marriott and Wright, 1993; Balin 2000).
The continental deposits of the
Punta San Andrés Alloformation (Plio-Pleistocene) crop-out in the marine cliffs
of south-eastern Buenos Aires Province (Mar
del Plata City, Argentina).
Deposits of this unit have been assigned to different sub-environments such as
floodplains (tabular and wedge-like silty sandstones), fluvial channels (mono
and multi-episodic ribbon sandstones) and temporary water bodies (tabular,
laminated silty sandstones). The aggradation rate of the floodplain deposits in
which palaeosols developed was never calculated. The aim of this presentation is
to provide information about this important feature in order to contribute to
the better understanding of the sedimentary dynamics and the influence of allogenic
factors in the accumulation of this unit.
As the Punta San Andrés palaeosols
are overlain by sheet-flood deposits, their superficial horizons have been
eroded. Therefore, a proper classification of these palaeosols is difficult to establish.
However, two main groups can be differentiated based on the sub-superficial
horizons identified. The first group is characterized by poorly developed Bw
horizons (2-2.5 m)
with typical, concentric, nucleic and amiboidal strongly impregnated Fe-nodules
(0.2-0.5 mm),
evidence of root activity (micro and mesoscopic channels with scarce clayey
coatings and hypo-coatings) and weakly developed subangular blocky peds (10-50 mm). When identified,
b-fabric is porostriated around a plane (slickensides) and granostriated. The
nature of this profile seems to be cumulic since identified Bw horizons are composed
of three or four sheet-flood pulses (30-40 cm each) affected by pedogenesis. The second
group is dominated by strongly developed calcic horizons (Bk), 1.5-2 m thick, with stage III (30-125
µm micritic peloids, micritic coated grains and 3-5 cm nodules), IV (6-8 cm laminated horizon with 0.5-1 mm micritc laminae) and V
(brecciation) morphologies. As carbonatization is rather intense, primary
structures can not be identified and the nature of these profiles can not be
assigned.
Even when the decapitated
nature of the palaeosol profiles makes estimation of aggradation rates
difficult, other characteristics such as palaeosols maturity and type can be
used to estimate a relative aggradation rate. In the Punta San Andrés case,
aggradation rates could have oscillated between periods of 2-5 mm/y and periods of less than 0.2-0.5 mm/y
aggradation rates (Marriott and Wright, 1993; Balin 2000).
The Punta San Andrés palaeosols
show a cyclic vertical alternation between Bw and Bk horizons, indicating
cyclic variations between relatively high aggradation rates (Bw horizons) and
lower aggradation rates (Bk horizons). The combination of these observations
with a facies and architectural analysis of the litosomes could help to develop
a model for sedimentation in the area and could provide information on whether
alternation between palaeosol types is due to climatic instability or other
allogenic factors.
Bibliography
Balin, D. F. 2000. Calcrete morphology and karst development in the Upper Old Red Sandstone
at Milton Ness, Scotland. In: Friend,
P. F. & Williams, B. P. J. (eds). New Perspectives on the Old Red
Sandstone. Geological Society, London,
Special Publications, 180, 485-501.
Marriott,
S. and Wright, V. P. 1993. Palaeosols as indicators of geomorphic stability in
two Old Red Sandstone alluvial suites, South Wales.
Journal of the Geological Society, London,
150: 1109-1120.