Download PDF - Speleogenesis

IMPLICATIONS OF HYPOGENIC TRANSVERSE SPELEOGENESIS
Northern Iraq. A karst-related origin is demonstrated
for the main sulfur deposits of northern Iraq (Jassim et al.,
1999). The brief description that follows is derived from
the cited work. This sulfogenic province is associated with
the Middle Miocene Fatha Formation, which contains
gypsum and/or anhydrite interbedded with carbonates,
marls and claystones. The Fatha Formation contains
aquifers in its carbonate beds and overlies the major
aquifer in the oil-bearing Lower Miocene carbonates of the
Euphrates-Jeribe Formation (Figure 58). The main deposits
(Lazzaga, Mishrag and Fatha) are located along the course
of the Tigris River, which partly incised into the Miocene
sequence, created a regional piezometric low and induced
upward discharge from the underlying confined system.
Sulfur mineralization is mostly restricted to the lower
member of the Fatha Formation. Isotopic signatures of
sulfur are consistent with the microbial formation of the
source H2S, and calcite that replaces gypsum inherits a
13
C-depleted isotopic composition from hydrocarbons.
The model for the sulfur origin suggested in Jassim et
al. (1999; Figure 58) invokes sulfur accumulation in
cavities, dissolved in a gypsum bed that averages 10 m
thick and is sandwiched between carbonate beds
conducting lateral groundwater flow. Mineralization
concentrates in zones where rising hydrocarbon-bearing
waters from the Lower Miocene carbonates mix with
lateral and downward influxes of oxygen-bearing water.
According to this model, the cavity zone experiences
alternating reducing/oxidizing conditions in response to the
fluctuating rainfall, allowing for alternating reduction of
dissolved sulfate and oxidation of H2S to accumulate
epigenetic calcite and sulfur. The model does not specify a
speleogenetic style for the formation of “a cavity” depicted
in Figure 58, but another work of Jassim et al. (1997)
interprets gypsum karst in the region in conventional terms
of “descending” surface-derived recharge and unconfined
systems. To fit with the stratabound occurrence of the
sulfur ore, the pattern of ore-hosting cave porosity should
be laterally pervasive. This can be produced in the given
hydrostratigraphic conditions only through confined
transverse speleogenesis. The hydrostratigraphic
arrangement described above seems very favorable to
supporting confined speleogenesis and the generation of
maze caves in the gypsum bed. In this case, the model for
the origin of sulfur deposits in Northern Iraq will be
largely like the above described model for the western
Ukraine, suggesting the critical role of speleogenesis.
Delaware Basin, West Texas. Bioepigenetic sulfur
deposits in the Delaware Basin are associated with the
thick evaporitic Castile, Salado and Rustler formations of
Upper Permian (Ochoan) age (see Figure 45 for location
and stratigraphy). The evaporitic sequence conformably
lies on the Lamar Limestone member of the hydrocarbonbearing
aquiferous Bell Canyon Formation, which is
Figure 58. The conceptual genetic model for sulfur deposits of
northern Iraq (from Jassim et al., 1999). Black arrows indicate
rising flow and white arrows indicate influx of oxygen-bearing
water.
composed of limestones, sandstones and marls, a basinal
equivalent of the Capitan Formation. The Castile
Formation is 200 to 600 m thick, has a basal limestone
member, but is mainly composed of anhydrite and gypsum,
with minor halite interbeds largely removed by dissolution
and substituted by homolithic breccia. The Salado
Formation varies in thickness from 30 m at the western
edge to 760 m in the center of the basin and is composed
of sulfates and halite. The upper part of the Salado
Formation contains most of the known sulfur deposits in
the region. The Rustler Formation, 30-200 m in thickness,
includes alternating siltstone, limestone, gypsum and
dolomite. Above the Permian sequence lies the Dewey
Lake Formation, composed of mudstone and siltstone.
Predominantly clastic cretaceous sediments overly the
Permian rocks through part of the basin.
All major works on the geology and origin of sulfur
deposits in the Delaware basin underscore their close
relationship with karst features, particularly with crossformational
features that allowed penetration of
hydrocarbons with upward flow across the thick evaporites
(e.g. Anderson and Kirkland, 1980; Kirkland and Evans,
1980; Miller, 1992; Wallace and Crawford, 1992). Of
particular relevance are those vertical karst structures that
extend from the basal limestone/sandstone aquifer. These
include masses of epigenetic calcite (either barren or
sulfur-bearing) called “buttes” or “castiles,” which form
exhumed hills on the eroded surface of the evaporites, and
heterolithic breccia chimneys (pipes). Anderson and
Kirkland (1980) recognized that density-driven convection
was the important mechanism for deep dissolution beneath
and across the evaporites, with fresh water from the basal
aquifers rising into the evaporites and developing crossformational
cavities, and brines returning to the aquifer to
ultimately outflow through it. At some point, such cavities
collapse to form breccia chimneys, but they persist to act as
91