Figure 1.
Location of the Stone City Bluff study area on the Texas Gulf Coastal Plain [59].
Figure 2.
Geologic Atlas of Texas, Austin Sheet, Burleson and Brazos Counties [60]. Stone City Bluff and Stone #1core located. Wilcox Group (Ewi); Crockett Fm., also known as the Cook Mountain Fm. (Ecm); and Caddell Fm. of the Jackson Group (Eca).
Figure 3.
Middle Eocene stratigraphic section includes Stone City Member, Crockett Formation, Claiborne Group (modified after Yancey [6]).
Figure 4.
Stone City Bluff outcrop photos, the MGB is green and nearly vertical, A) looking west, upstream (the subject in this photograph has given written informed consent, as outlined in the PLOS consent form, to publication of their photograph), B) looking east, downstream.
Figure 5.
Upper units of the Stone City Member, sample number and location (modified after Stenzel [2]).
Figure 6.
Stratigraphic chart and related lithostratigraphy of the Brazos River Valley, major sequence boundaries (solid lines) and maximum flooding surfaces (dashed lines), identified by S = seismic; W = well logs; O = outcrop (modified after Davidoff & Yancey [7]).
Figure 7.
Diagrammatic verdine facies model depicting the idealized paleoenvironment at a tropical river mouth (modified after Odin [16]).
Figure 8.
Photomicrographs from central MGB, A) TX-9, B) TX-8, cp = clay pellet, hp = heterogeneous pellet, sh = shell, Q = quartz.
Figure 9.
Top of MGB concretionary burrow fill, photomicrographs and QEMSCAN (false color) images, cp = clay pellets, hp = heterogeneous pellets, pink = apatite cement.
Figure 10.
X-ray diffraction patterns of oriented, 2 µm clay fraction, from bulk sample TX-18, a mixture of 7 and 14Å clays (air dried, glycolated and heated to 375°).
Figure 11.
X-ray diffraction pattern of an unoriented powder mount of concentrated clay pellets, TX-MGB.
Eleven labeled peaks were indexed in unit cell calculation for odinite-1M. Unlabeled peaks are quartz. Peak at 2θ = 59.73 is quartz overlapped by odinite.
Table 1.
Unit cell calculation from XRD data (odinite-1M (monoclinic)).
Figure 12.
Chemical analyses (apfu), published [16], [24] and MGB clay pellets, used to calculate mineral formula.
Table 2.
Representative analyses from MGB clay pellets, 11 each from the central MGB and concretionary burrow fill at the top of the MGB.
Table 3.
Maximum, minimum, mean and standard deviation of chemical analyses from MGB pellets and published odinite.
Figure 13.
MGB clays plotted within the compositional field of iron-rich clays from the Claiborne Group.
Open squares (green) are from the MGB and solid squares are glauconitic minerals from the MGB. Open circles are published analyses of odinite [16], [24]. The range of compositions of clay in pellets from the Claiborne Group is indicated by the large oval [11], [61]. The field of berthierine is from Brindley [61].
Figure 14.
Plot of isomer shift vs. quadrupole splitting (in mm/s).
A) Clays from the MGB (this study). B) MGB clays compared to published analyses of glauconite.
Table 4.
Mössbauer parameters of 7Å clay mixture.
Figure 15.
Central MGB QEMSCAN images with specific mineral area %, sample TX-18.
Figure 16.
Central MGB QEMSCAN images with specific mineral area %, sample TX-9.
Figure 17.
Top of MGB QEMSCAN images with specific mineral area %, sample TX-17.
Table 5.
Concretionary burrow fill, MGB top, siderite and mole percent carbonate end-members calculated with CaCO3.
Table 6.
Concretionary burrow fill, MGB top, a fine-grained mixture of apatite and one or more alumino-silicates.