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The Geology of
Tarrant County
" The formation consists of blackish and bluish shales, with
seams of arenaceous and shelly limestone, and weathers into a black, waxy,
carbonaceous, treeless, rolling upland soil. The formation is stated to be
about 500 feet thick between Arlington and the White Rock escarpment west
of Dallas; of this thickness about half occurs in southeastern Tarrant
county. "
The Geology of Dallas County
" The Eagle Ford
formation Group consists mostly of clay-shale.
Within the county it has an average thickness of about 475 feet. Hill
(1887, p. 296, 298) named the formation for exposures around the small
settlement of Eagle Ford, which is situated on the south side of the
Trinity north of Arcadia Park"
" The basal beds of the Eagle Ford, exposed, short distances
beyond the western boundary of the county, have been described by Taff
(1893, p. 292), Moreman (1927, p. 90), and Adkins (1932, p. 425-426). At
the base is a thin conglomerate layer which overlies the upper Woodbine.
This is succeeded upward by sandy clay and shale with partings of impure
limestone and calcareous concretions. The material becomes less sandy
upward in the section, the sand persisting in recognizable quantity up to
levels between 15 and 90 feet above the base. As the sand decreases in
quantity the shale changes color, from brownish and brownish-gray at the
bottom to dark gray, sometimes with a bluish cast, higher in the
section."
" Moreman (1927, p. 90) notes that the shale becomes highly
calcareous and almost white beginning at a horizon approximately 150 feet
above the base. These light-colored calcareous shales are around 50 feet
thick, and grade upward into dark gray and bluish gray shales which are
about 200 feet thick. At many horizons these shales contain concretions
and lentils of dense gray limestone and ferruginous claystone."
" Flaggy limestone [Kamp Ranch] breaks the shale sequence 75
feet below the top of the Eagle Ford at Arcadia Park. These beds, well
exposed in cuts along the Dallas-Fort Worth highway at the western edge of
the town, have a thickness of one foot. They consist of gray to brownish,
brown-weathering limestone containing considerable clay, silt and fine
sand, and bedded in units from a fraction of an inch to three inches
thick. On the rough and, irregular surfaces of bedding are numerous
fossils including small ammonites, pelecypods, fish teeth [Ptychodus], and
worm (?) castings. Other portions of the rock are made of calcareous
shell fragments. The fossils are not uniformly distributed through the
flags, however, and some layers appear to be barren, as Scott (1940, p.
312-313) has observed."
" Flaggy material of the same character as that described above
is exposed along the west side of Ledbetter Road about one mile south of
Arcadia Park. The main flaggy layer is slightly over one foot thick, but
for six inches above the top there are alternating layers of clay and
impure lime-stone, the latter in units averaging a quarter of an inch
thick. Other exposures of the flaggy layers may be seen along Mountain
Lake pike near its intersection with Blue Cut road. Between the flaggy
layers and the top of the formation is a thickness of 75 feet, mostly dark
gray to black shale. These upper beds are well exposed in the railroad cut
directly south of the Dallas-Fort Worth highway between Arcadia Park and
Chalk Hill, in the embankments along the same highway at Chalk Hill, and
in the quarry of the Trinity Portland Cement Co. "
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Austin Chalk
Woodbine Fm.
