Geologic Map of the Lost Mine Trail Area - Confex · Oligocene Boot Rock Member ... Eocene Ash...
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Quaternary Alluvium – Deposits of fluid-moved sediments composed of rounded cobbles and boulders. Quaternary Colluvium – Talus slopes containing thick deposits of blocky angular rocks and debris, mostly on steep bases of large cliff-forming outcrops. Oligocene Lava Dome – White to blueish-green fine-grained rhyolite. Trachytic texture with small sanidine phenocrysts (Or24-39), quartz, arfvedsonite, and iron-titanium oxides. Caprock of Casa Grande Mountain is composed entirely of this rock, as well as feeder dikes on the eastern and western flanks of Casa Grande. Feeder dikes show less oriented texture, coarser groundmass, and alteration. Oligocene Intrusive Dikes – Fine-grained porphyritic trachytes to rhyolites of varying color and texture with sanidine phenocrysts (Or24-40), arfvedsonite, sparse quartz, and iron-titanium oxides. Texture varies from trachytic to a slightly oriented groundmass. Rocks are highly altered in certain localities. Dikes at the end of Lost Mine Trail are finer grained than those on Casa Grande and show a difference in groundmass texture, but composition is consistent, including Or content of feldspar phenocrysts. A set of dikes containing a dark grey to blue matirx with larger, up to 1cm phenocrysts of micro-perthite is also present in the area. Oligocene Boot Rock Member – Brown to maroon-grey densely welded lithic ash flow tuff and rhyolite. Multiple interbedded units of pyroclastic deposits with sediment, ash, and lithic fragments that vary in matrix color from grey, green, to brown. Large blocks in densely welded tuff matrix can be over 2 meters in length at the end of Lost Mine Trail. Unit varies greatly within mapping area, and thickens to the northeast on Casa Grande. Oligocene Large Intrusion/Lava Dome - White to blueish fine-grained rhyolite with small phenocrysts of alkali feldspar up to 4mm. Equigranular texture with quartz, arfvedsonite, apatite, and aegirine-augite. Thought to be associated with the other large intrusional bodies that surround the Basin area. Barker (2007) presents valid evidence about this perceived intrusion being a large lava dome. Oligocene Pine Canyon Rhyolite – Grey to brown ash flow tuff and rhyolite. Fine-grained with small alkali feldspar phenocrysts. Trachyte inclusions. Only present in a steep cliff on the east wall of Juniper Canyon in mapping area. Oligocene Bee Mountain Basalt – Dark grey, black, to brown fine-grained basalt to basaltic trachyandesite. Secondary minerals fill in vesicles in upper portion of the unit. Microphenocrysts of olivine and clinopyroxene visible. Thickness varies from a few meters around the top of Lost Mine Trail to up to 90 meters thick in other parts of the mapping area. Unit thickens to the east in mapping area, but is eroded at the angular unconformity on the upper portion of Lost Mine Trail. Eocene Ash Spring Basalt – Dark purple, brown, to grey, porphyritic basaltic to trachyandesitic rock with plagioclase phenocrysts up to 1.5 centimeters. Vesicular in places. Very fine-grained matrix with olivine and pyroxene microphenocrysts. Unit up to 27 meters thick in mapping area. Weathers to a brownish grey, to green, blocky soil. Eocene-Oligocene Chisos Formation Undifferentiated Tuffaceous Sediements - Multiple alternating beds of fine to course grained sandstones, siltstones, limestone, including the large unit of grey sandstone conglomerate previously referred by Udden (1907) as the Crown Conglomerate. Beds are deposited between multiple mafic lava flows and are intermittently tuffaceous. GEOLOGIC MAP OF THE LOST MINE TRAIL AREA, BIG BEND NATIONAL PARK, TEXAS By NO VERTICAL EXAGERATION ON ALL SECTIONS UTM NAD 1983 Zone 13 N References Barker, D. S. 2007. Endogenous and exogenous plutons. The Canadian Mineralogist, v. 45, p. 63-70. Udden, J. A. 1907. A sketch of the geology of the Chisos country, Brewster County, Texas. Bulletin of the University of Texas, v. 93 no. (11). Seth Sonnier Oligocene Intrusive - Single dike found with spherulitic devitrified matrix and large quartz phenocrysts. Geology by Seth Sonnier 2012-2013 A’ A
Transcript of Geologic Map of the Lost Mine Trail Area - Confex · Oligocene Boot Rock Member ... Eocene Ash...
Quaternary Alluvium – Deposits of �uid-moved sediments composed of rounded cobbles and boulders.
Quaternary Colluvium – Talus slopes containing thick deposits of blocky angular rocks and debris, mostly on steep bases of large
cli�-forming outcrops.
Oligocene Lava Dome – White to blueish-green �ne-grained rhyolite. Trachytic texture with small sanidine phenocrysts (Or24-39), quartz,
arfvedsonite, and iron-titanium oxides. Caprock of Casa Grande Mountain is composed entirely of this rock, as well as feeder dikes on the
eastern and western �anks of Casa Grande. Feeder dikes show less oriented texture, coarser groundmass, and alteration.
Oligocene Intrusive Dikes – Fine-grained porphyritic trachytes to rhyolites of varying color and texture with sanidine phenocrysts (Or24-40), arfvedsonite, sparse quartz, and iron-titanium oxides. Texture varies from
trachytic to a slightly oriented groundmass. Rocks are highly altered in certain localities. Dikes at the end of Lost Mine Trail are �ner grained than those on Casa Grande and show a di�erence in groundmass texture, but composition is consistent, including Or content of feldspar phenocrysts.
A set of dikes containing a dark grey to blue matirx with larger, up to 1cm phenocrysts of micro-perthite is also present in the area.
Oligocene Boot Rock Member – Brown to maroon-grey densely welded lithic ash �ow tu� and rhyolite. Multiple interbedded units of pyroclastic deposits with sediment, ash, and lithic fragments that vary in matrix color from grey,
green, to brown. Large blocks in densely welded tu� matrix can be over 2 meters in length at the end of Lost Mine Trail. Unit varies greatly within
mapping area, and thickens to the northeast on Casa Grande.
Oligocene Large Intrusion/Lava Dome - White to blueish �ne-grained rhyolite with small phenocrysts of alkali feldspar up to 4mm. Equigranular texture with quartz, arfvedsonite, apatite, and aegirine-augite. Thought to
be associated with the other large intrusional bodies that surround the Basin area. Barker (2007) presents valid evidence about this perceived intrusion
being a large lava dome.
Oligocene Pine Canyon Rhyolite – Grey to brown ash �ow tu� and rhyolite. Fine-grained with small alkali feldspar phenocrysts. Trachyte
inclusions. Only present in a steep cli� on the east wall of Juniper Canyon in mapping area.
Oligocene Bee Mountain Basalt – Dark grey, black, to brown �ne-grained basalt to basaltic trachyandesite. Secondary minerals �ll in
vesicles in upper portion of the unit. Microphenocrysts of olivine and clinopyroxene visible. Thickness varies from a few meters around the top of Lost Mine Trail to up to 90 meters thick in other parts of the mapping area.
Unit thickens to the east in mapping area, but is eroded at the angular unconformity on the upper portion of Lost Mine Trail.
Eocene Ash Spring Basalt – Dark purple, brown, to grey, porphyritic basaltic to trachyandesitic rock with plagioclase phenocrysts up to 1.5 centimeters. Vesicular in places. Very �ne-grained matrix with olivine
and pyroxene microphenocrysts. Unit up to 27 meters thick in mapping area. Weathers to a brownish grey, to green, blocky soil.
Eocene-Oligocene Chisos Formation Undi�erentiated Tu�aceous Sediements - Multiple alternating beds of �ne to course grained sandstones, siltstones, limestone,
including the large unit of grey sandstone conglomerate previously referred by Udden (1907) as the Crown Conglomerate. Beds are deposited between multiple
ma�c lava �ows and are intermittently tu�aceous.
GEOLOGIC MAP OF THE LOST MINE TRAIL AREA, BIG BEND NATIONAL PARK, TEXASBy
NO VERTICAL EXAGERATION ON ALL SECTIONS
UTM NAD 1983Zone 13 N
References
Barker, D. S. 2007. Endogenous and exogenous plutons. The Canadian Mineralogist, v. 45, p. 63-70. Udden, J. A. 1907. A sketch of the geology of the Chisos country, Brewster County, Texas. Bulletin of the University of Texas, v. 93 no. (11).
Seth Sonnier
Oligocene Intrusive - Single dike found with spherulitic devitri�ed matrix and large quartz phenocrysts.
Geology by Seth Sonnier2012-2013
A’A
