Download PDF - Speleogenesis

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NCKRI Special Paper No. 1
The system occurs in Lower Cretaceous
forereef/reef/backreef limestones overlain by Upper
Cretaceous argillaceous limestone and shale deposits
(Figure 54) that cover much of northeastern Mexico. The
Laramide orogeny and uplift exhumed the younger strata.
During the Tertiary, igneous activity had a significant
imprint on the regional geomorphology. The Villa Aldama
volcanic complex, located within 5 km of El Zacaton,
consists of Pliocene and Pleistocene lava flows and shield
volcanoes, with the most recent igneous rocks dated at 250
ky (Gary and Sharp, 2006).
Sistema Zacatón is believed to have developed under
the direct influence of Pleistocene volcanic activity, which
provided the thermal gradient, CO2 and H2S to drive
dissolution mechanisms at increased rates (Gary and
Sharp, 2006). The inferred evolutionary model implies that
the development of the karst system began much
earlier, since a deep groundwater flow system with
Figure 54. Geologic map of northeastern Mexico showing
the location of the deep karst shafts in the region (shown
as bull's-eye circles). Major depositional and structural
features are also represented (from Gary and Sharp,
2006).
recharge at upland areas was established by the
early Tertiary. It continued through the middle to
late Tertiary under the influence of intermittent
intrusive volcanic activity. Pleistocene volcanism
accelerated and focused speleogenesis in the area.
Variations in connections with the deep flow
system and in the degree of interaction with
shallow groundwaters and surface waters account
for varying geochemical characteristics of water in
different sinkholes. Water in El Zacatón is
undersaturated with calcite.
The primary trend of sinkholes/pits is roughly
linear, north to south, coinciding with fractures
observed in the area and the axial trace of the
Tamaulipas Arch anticline (Figure 54). There is a
secondary E-W trend in fracture and sinkhole
pattern. El Zacatón's lateral extent and pattern of
cavities at depth is unknown. It is also uncertain
whether the shafts were formed due to collapses
over large chambers at depth or as dissolution
features of rising flow. Gary and Sharp (2006)
believe that the sinkholes formed due to collapse.
Another known deep phreatic shaft, 392-m deep
Pozzo del Merro near Rome, Italy, shows the
morphology of a rising shaft (Figure 27).
Numerous maze caves are known from several
large basins in Brazil, formed in the predominantly
carbonate Precambrian Una Group in the São Francisco
Craton. Outstanding examples described from the Campo
Formoso area are the 125 km long Toca da Boa Vista and
28 km long Toca da Barriguda caves, both developed in
the carbonate sequence of the Salitre Formation (Auler and
Smart, 2003). The caves show no genetic relationships to
the surface, display many features characteristic of
hypogenic caves and no vadose features. The cave plans
exhibit densely packed, joint-controlled patterns,
predominantly network, with some larger passages and
chambers (Figure 55). The cited work mentions continuous
phreatic dissolutional features that can be traced up to the
cave entrances, and suggests that the cave passages once
extended above their present surface elevation, being
intersected by denudational lowering of the surface.
Auler and Smart (2003) suggested a hypogenic origin
for these caves, but their connotation of “hypogenic” is