Root calcrete formation on Quaternary karstic surfaces of Grand Cayman

A.

Description

Abstract
The rugged karst terrain developed on the dolostones of the Miocene Cayman Formation (Fm) on Grand Cayman includes numerous large cavities that formed through the activity of tree roots. The surfaces of those cavities are coated with laminated calcrete crusts up to 8 cm thick that are formed of an alteration zone, an accretionary zone, and final infill of the cavities. These crusts are formed of various laminae, including dolostone with root traces, alveolar septal structures, peloids, micritic and microsparitic laminae, micrite with bioclasts, and pisoliths. Features such as microborings, spores, needle-fiber calcite and micro-rods are common in all parts of the calcrete crust. Calcrete formation was initiated as the roots and associated microorganisms generated the cavities. Later on trapping and binding processes and organically induced precipitation of carbonate allowed the formation of the accretionary (mostly laminar) part of the calcrete. The last phases of crust formation took place when ponded waters filled the cavities. The calcrete crusts developed on the Cayman Formation dolostones record a very specific setting for calcrete formation and constitute a good example of non-horizontal calcrete crusts.

Sujets

• Root
• Karst
• Cayman Islands
• Quaternary

Informations

• Publié par: erevistas
• Publié le: 01 janvier 2007
• Nombre de lectures: 16
• Langue: English
• Poids de l'ouvrage: 1 Mo

Extrait

Geologica Acta, Vol.5, Nº 1, 2007, 77-88
Available online at www.geologica-acta.com
Root calcrete formation on Quaternary karstic surfaces of
Grand Cayman
1 2
A.M. ALONSO-ZARZA and B. JONES
1 Dpto. Petrología y Geoquímica, Facultad de CC. Geológicas, Universidad Complutense de Madrid
28040 Madrid, Spain. E-mail: [e-mail]
2 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton
Alberta, Canada T6G 2E3. E-mail: [e-mail]
ABSTRACT
The rugged karst terrain developed on the dolostones of the Miocene Cayman Formation (Fm) on Grand Cay-
man includes numerous large cavities that formed through the activity of tree roots. The surfaces of those cavi-
ties are coated with laminated calcrete crusts up to 8 cm thick that are formed of an alteration zone, an accre-
tionary zone, and final infill of the cavities. These crusts are formed of various laminae, including dolostone
with root traces, alveolar septal structures, peloids, micritic and microsparitic laminae, micrite with bioclasts,
and pisoliths. Features such as microborings, spores, needle-fiber calcite and micro-rods are common in all
parts of the calcrete crust. Calcrete formation was initiated as the roots and associated microorganisms genera-
ted the cavities. Later on trapping and binding processes and organically induced precipitation of carbonate
allowed the formation of the accretionary (mostly laminar) part of the calcrete. The last phases of crust forma-
tion took place when ponded waters filled the cavities. The calcrete crusts developed on the Cayman Formation
dolostones record a very specific setting for calcrete formation and constitute a good example of non-horizontal
calcrete crusts.
KEYWORDS Calcretes. Root. Karst. Cayman Island. The Caribbean. Quaternary.
INTRODUCTION (a) subaerial exposure (Esteban and Klappa, 1983), (b)
semiarid to arid climate climatic conditions (Goudie,
Calcretes are near surface, terrestrial, accumulations 1983; Alonso-Zarza 2003), (c) stable geomorphic surfaces
of predominantly calcium carbonate, that vary from pow- (Machette, 1985), and/or (d) non-active areas in alluvial
dery to nodular to highly indurated forms. They result systems (Wright and Alonso-Zarza, 1990). Although
from cementation and the introduction of calcium carbon- found in many different geological settings, calcretes are
ate into soil profiles, bedrock, and sediments (Goudie, typically associated with soil profiles in surficial or sub-
1973; Wright and Tucker, 1991). Also known as caliches, surficial locations. It should be noted, however, that non-
they have been the subject of numerous studies over the pedogenic and phreatic calcretes have also been recog-
last two decades because they are important indicators of nized (Arakel, 1991; Mack et al., 2000). Calcrete
© UB-ICTJA 77A.M. ALONSO-ZARZA and B. JONES Root calcrete formation-Grand Cayman Is.
formation is controlled by the interplay of a wide range of Oligocene), the Cayman Fm (Middle-Late Miocene), and
biological and physico-chemical processes – a fact that is the Pedro Castle Fm (Late Pliocene) (Jones, 1994).
amply demonstrated by the wide variety of microfabrics
associated with calcretes (Wright, 1994). Calcrete micro- Grand Cayman has a relief of less than 20 m with
fabrics commonly preserve evidence of the role that vari- large parts of the island being less than 10 m above sea
ous organisms have played in their formation. level. Hard, finely crystalline dolostones that belong to
the Cayman Fm are exposed over most of the eastern half
Calcretes developed on different Caribbean island of Grand Cayman (Jones, 1994, his fig. 2.3). In contrast,
record the subaerial processes that formed in association much of the western half of Grand Cayman has lime-
with recent soil formation. In this paper we examine the stones of the Ironshore Fm and/or Holocene swamp
development of calcrete on the dolostones of the Cayman deposits at the surface. In this area, hard dolostones of the
Fm on Grand Cayman. In so doing it is possible to assess Cayman Fm are restricted to an elevated ridge that paral-
the roles of pedogenic and biological processes in calcrete lels the south coast along with isolated exposures in the
formation and their significance to subaerial diagenesis southwest corner and at Hell. Surface exposures of the
processes. Much of the calcrete development is found in Pedro Castle Fm are restricted to the south part of the
large, steep to vertical karstic-tree root cavities. They island in the vicinity of Pedro Castle (Jones, 1994, his fig.
offer a unique opportunity to examine calcrete develop- 2.3). When exposed at the surface, the hard, finely crys-
ment on non-horizontal surfaces that are located within talline dolostones of the Cayman Fm are typically charac-
the host rock rather than on its upper surface. The close terized by rugged phytokarst development (Folk et al.,
association of laminar calcretes with roots has led to their 1973; Jones, 1994) that is characterized by rugged, sharp-
designation as rhizogenic calcretes (Wright et al., 1995) edged pinnacles separated from each other by deep
or rootcretes (Jones, 1992). Only exceptional develop- crevices and potholes that are commonly less than 1 m
ment of calcretes within oblique fractures or root cavities wide (Jones, 1994, his fig. 2.12). The little soil that is pre-
has been considered (Calvet and Juliá, 1983). sent is generally concentrated in the potholes and crevices.
Areas with extensive soil cover are rare and localized.
GEOLOGICAL CONTEXT Most of the eastern part of Grand Cayman, which is
the least developed part of the island, is covered with
The central part of each of the Cayman Islands is formed dense tropical vegetation despite the sparse, poorly deve-
of massive Tertiary limestones and dolostones that belong to loped soil. Hardwood forests that included mahogany, red
the Bluff Group (Jones, 1994; Fig.1). This core is uncon- birch, ironwood, cedar, and fig originally covered much
formably onlapped by the Ironshore Fm, which is formed of of the eastern part of Grand Cayman. Today, much of the
poorly lithified limestones that were deposited at various original forest is restricted to small patches and other areas
times throughout the Pleistocene (Jones, 1994; Vézina et al., have been replaced by thatch palms, manchineal, and log-
1999). The Bluff Group is formed of the Brac Fm (Late wood, vines, climbing cacti, and maidenplum (Bradley,
FIGURE 1 Map showing the location of Grand Cayman Island and the study area.
Geologica Acta, Vol.5, Nº1, 2007, 77-88 78A.M. ALONSO-ZARZA and B. JONES Root calcrete formation-Grand Cayman Is.
1985; Brunt, 1994). Although some of these plants are associated with trees that have large tap-roots, numerous
rooted in the soils in the potholes, others are rooted directly secondary roots, and large masses of rootlets and root
into the bedrock. Some of the large trees, for example, hairs (Fig. 2). Conversely, the smaller roots of other
have roots that extend down into the bedrock for in excess plants (e.g., bushes, grasses) only give rise to small cavi-
of 6 m. Presumably, the large taproots penetrate down to ties that have a relatively simple tubular morphology.
areas where they can access the groundwater. Many of the root cavities are filled with brecciated frag-
ments of the bedrock and various accumulations of terra
The rocks around the plant roots have been subjected rossa soils.
to considerable diagenetic alteration (e.g., Jones, 1989).
In many areas, it is readily apparent that the roots have
brecciated the host rock to the extent that large isolated DESCRIPTION OF ROOTCRETE
fragments of the dolostone are completely enmeshed by
roots and root hairs. The size and morphology of the On the Cayman Islands, rootcretes are common fea-
cavities associated with the plants are highly variable tures of the dolostones of the Cayman Fm. The highly
and a function of the size and morphology of the root irregular, sharp to diffuse boundary between the rootcrete
system. Cavities up to 2 m in diameter are commonly and the dolostone (Fig. 2A) indicates that the host rock
FIGURE 2 Field photographs of root
calcretes in the Cayman Formation,
northeast part of Grand Cayman. A) Root
calcrete surrounding tree roots in dolo-
stones in upper part of Cayman Forma-
tion. B) Brown, thinly laminated root cal-
crete lining cavity created by tree roots.
C) Cavity facing surface of root calcrete
showing small grooves that were once
filled by tree rootlets and root hairs. D)
Root cavity in dolostone lined with man-
ganese-rich root calcrete (RC) and filled
with lithified terra rossa (TR).
Geologica Acta, Vol.5, Nº1, 2007, 77-88 79A.M. ALONSO-ZARZA and B. JONES Root calcrete formation-Grand Cayman Is.
underwent dissolution and/or corrosion which are the calcretes of the Madrid Basin and interpreted as calci-
result of the phytokarst development indicated above. The fied root cells (Alonso Zarza et al., 1998a). Some fila-
rootcrete typically displays a wavy profile and the cal- ments occur close to the calcified cells. Alveolar septal
crete commonly penetrate to different depths, within the structures are also present around the holes.
host rock (Fig. 2B and 2C). The walls of the root cavities
are commonly lined with laminated crusts, up to 2 cm Indurated dolostone with root traces
thick, that have been refer