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Abstract
Chapter from: M
63: Unconformities and Porosity in Carbonate Strata
Edited By
D.A. Budd, A.H. Saller, and P.M. HarrisAuthor:
F. Jerry Lucia Carbonate Reservoirs
Published 1995 as
part of Memoir 63
Copyright © 1995 The American Association of Petroleum
Geologists. All Rights Reserved. |
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Chapter 14
*
Lower Paleozoic
Cavern Development, Collapse, and Dolomitization, Franklin Mountains, El
Paso, Texas
F. Jerry Lucia
Bureau of Economic Geology
The University of Texas
at Austin
Austin, Texas, U.S.A.
*
ABSTRACT
The superb exposures of the Lower Ordovician
El Paso Group in the Franklin Mountains, El Paso, Texas, provide an excellent
opportunity to investigate the effects of unconformities on porosity and
permeability of carbonate rocks. Unconformities at cycle, sequence, and
supersequence boundaries represent time gaps ranging from thousands to
millions of years. Unconformities at cycle and sequence boundaries are
marked by tidal-flat facies and reflux dolomitization. No significant karsting
is found at these boundaries.
A large cavern system was developed in
the upper 300 m (1000 ft) of the El Paso Group during the 33 m.y. time
gap marked by the supersequence boundary between the Lower Ordovician El
Paso Group and the Upper Ordovician Montoya Group. In the upper 75 m (250
ft), the El Paso caverns were tabular and horizontal and were formed near
the phreatic-vadose interface. In the lower 225 m (750 ft), the caverns
were linear and vertical and were formed in the deep phreatic zone along
vertical fractures oriented N20°W and spaced 900 m (3000 ft) apart.
A stratiform dolomite unit separated the two cave systems. Collapse was
initiated during cave development and continued through Silurian time.
Collapse of the El Paso caverns formed large fracture systems and megacollapse
breccias 300 m (1000 ft) thick, 450 m (1500 ft) wide, and several kilometers
long. Collapse of the cavern roof produced brecciation and fracturing in
the overlying Montoya strata. Much of the breccia and adjacent country
rock was dolomitized by fluid migrating through the collapsed caverns after
Silurian time.
Cavern development, collapse, and dolomitization
of the El Paso and Montoya groups has completely altered the original porosity
and permeability distribution from one controlled by depositional patterns
to one controlled by diagenetic processes. Karst-related dissolution resulted
in cavernous porosity comprising up to 30% of some intervals. However,
infilling sediment and collapse during burial destroyed most of the cavernous
porosity by the end of Silurian time; by the end of Pennsylvanian time,
much
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