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