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