Geology Reference
In-Depth Information
Early load-induced fracturing in a prograding carbonate margin
DONALD F. MCNEILL and GREGOR P. EBERLI
Comparative Sedimentology Laboratory, Marine Geology and Geophysics, University of Miami,
4600 Rickenbacker Causeway, Miami, FL 33149, USA (E-mail: dmcneill@rsmas.miami.edu)
ABSTRACT
The description of early non-tectonic fracturing of carbonates is rare because
(1) fractures in young carbonates occur in areas not easily accessible, and (2) burial
and diagenesis often precludes the determination of the exact timing of (early) fractur-
ing in older rocks. The Bahamas Drilling Project core Clino recovered the upper part
of a young prograding platform margin containing a set of fractures interpreted to be
non-tectonic; formed by rapid loading of a clinothem package. Three intervals with
open and partially infi lled fractures were recovered in Clino, and those fractures are
closely related to the lithological variation in the slope sediments. Fracturing occurred
specifi cally within coarse, well-cemented skeletal deposits. These coarser deposits are
generally margin-derived sediments within the background sediment. The overlying
and underlying fi ner-grained sediments are signifi cantly less fractured. The fractures
occur within a 45-m-thick interval immediately below a surface of non-deposition
or erosion with a hiatus of >0.5 Myr. A proposed model suggests that early cementa-
tion of the skeletal sands, followed by rapid margin progradation, provided suffi cient
load to generate brittle fracturing (shear and extensional) in lithifi ed strata. Laboratory
deformation experiments, magnetic fracture-plane orientation and the timing of load
emplacement are consistent with load-induced fracturing. Fracture planes preferen-
tially dip westward which is parallel to the depositional slope. Similar early fracture
porosity can act as high permeability conduits for diagenetic fl uids and hydrocarbons
in platform margin and slope strata.
Keywords
Fracture, carbonate, load-induced, clinothem, Bahamas, porosity.
INTRODUCTION
et al
., 2001) framework has been produced for core
Clino. This extensive dataset allows a comprehens-
ive evaluation of the conditions that contributed
to early rock fracturing. The objectives of this
paper are to use the fracture inclination, fracture
orientation, fracture density and constraints on
fracture timing to illustrate a mechanism of early
fracturing. The hypothesis is that early diagenesis
and rapid progradation of the platform margin to
invoke fracturing. The mechanism of early frac-
turing in the model has implications for prefer-
ential fl uid exchange and might be a contributing
factor in the collapse of platform margins (Hine
et al
., 1992).
Early fracturing of lithifi ed carbonates is a rarely
reported phenomenon. Observation of fractures
in young carbonates is diffi cult because such fea-
tures generally occur in relatively inaccessible
near-surface settings. In older rocks, burial and
diagenetic history may preclude the identifi ca-
tion and the exact timing of fracturing, making
it diffi cult to determine that fracturing occurred
early
. The Bahamas Drilling Project core Clino
recovered, for the fi rst time, a complete section
of a young (Neogene) prograding platform margin
along the western margin of Great Bahama Bank
(Fig. 1). This core recovered a set of fractures in the
clinothem package, and they are described here.
These fractures are natural and clearly not drilling
induced (Kulander
et al
., 1990). Our interpretation
of the fractures benefi ts greatly from existing stud-
ies. A detailed lithological (Kenter
et al
., 2001),
stratigraphic (Eberli
et al
., 2001; McNeill
et al
.,
2001) and diagenetic (Melim
et al
., 2001; Swart
Stratigraphic setting of the fracture interval
Three intervals of open and partially infi lled frac-
tures were recovered in core Clino within mid-
Pliocene seismic sequences
e
and
f
, just below
their respective seismic sequence boundaries
(SSB) D and E (Fig. 2) (see Eberli
et al
. (2001) for
