"When I looked at the picture of Vaughn's house from across the creek, I noticed the rocks made a line all the way down to the creek. Then I looked at that map I found:
Attached [below] is a little piece of the map and a little piece of the Google Earth aerial view. It seems I read that some geological formations can be traced on aerial maps, and there does look like there might be a correlation between the edge of the Marble Falls limestone and the rock bluff behind Vaughn's house."
In the photo above (left), our little oval-shaped pond is featured in green, south of where where the upper yellow line bends. That same angle is represented in the second image (right), and the same angle is shown about where the blue creek makes its eastward bend. These lines (yellow in the left image and black in the right image) represent a fault line that appears to be somewhat responsible for the location of the creek.
The other important piece of information here is that the lavender shaded area west of that bend (and including most of our little property) represents the Marble Falls Limestone formation. The green shaded area (including the bluff just east of the creek) represents the Smithwick Formation. The MF Limestone is the lower formation. We read in one source, "At the top [of the Marble Falls limestone] the formation is sharply limited and the Smithwick shale succeeds it with perfect accordance in dip. The formation is named for the typical exposures at Marble Falls, Burnet County" [http://www.lib.utexas.edu/books/landscapes/publications/txu-oclc-301083/txu-oclc-301083-08.html]
(The black and white road on the geologic map above is CR 342-C.)
Here are the layers of the top portion of the Marble Falls Limestone:
| In Feet. | |
| Top, black shale. | |
| Thin-bedded black shale | 20 |
| Fine-grained dark-gray limestone | 15 |
| Thin-bedded and massive black limestone | 30 |
| Brownish-gray subcrystalline to crystalline limestone | 30 |
| Thin-bedded dark-gray or black fossiliferous limestone with Productus and crinoids | 4 |
| Fine-grained crystalline gray limestone | 4 |
| Fine grained gray limestone with molluscan fossils | 20 |
| Dense black or dark-gray limestone | 2 |
| Light and dark gray mottled limestone with chert nodules | 5 |
| Mottled gray limestones with crinoid stems 26 inches long | 5 |
| Limestone conglomerate | 1 |
| Gray limestone without chert | 2 |
| Gray cherty limestone | 8 |
| Black limestone with cherty layers | 8 |
| Black evenly bedded limestone with cherty layers | 9 |
| Massive gray crystalline limestone | 8 |
| Gray cherty limestone | 25 |
| Gray crystalline limestone | 6 |
| Fault. | |
| Irregular-bedded gray limestone | 17 |
| Black shaly limestone with chert layers and lenses | 22 |
| Massive gray limestone, with crinoid stems | 60 |
| Fault. | |
| Dark cherty limestone | 7 |
| Dove-colored limestone with black bands and lenses | 40 |
| Conglomerate | 20 |
| Ellenburger limestone. | ___ |
| 368 |
According to the USGS, the Smithwick Formation is dated as:
"Phanerozoic | Paleozoic | Carboniferous Pennsylvanian-Middle [Atoka]
Comment: (fr. Llano Sheet, Geol. Atlas of Texas, 1981) In type locality east of Marble Falls, claystone, siltst., and ss. Clayst. predom. laminated or burrowed, carbonaceous; illite with less than 15 percent quartz and muscovite silt; ss mostly contains rock frags. (sublitharenite of Folk) and minor but stratigraphically important sandy ls.-pebble conglomerate derived from the Marble Falls Limestone; sole marks and a wide variety of sedimentary structures present in sandstone; plant material abundant in ss and clayst.; fossils includesome plants, locally abdt. trilobites, brachiopods, and cephalopods, thickness about 300 ft. In Mason County, Smithwick consists of black shale with ls. beds in lower part, which is laterally gradational to Marble Falls Ls., lower boundary climbs stratigraphically westward."
Regarding the Smithwick formation, we read:
"The Smithwick shale, named from the old town of Smithwick, in Burnet County, consists of soft, very dark or nearly black carbonaceous shale in which are included a number of sandstone lentils. Being soft and easily disintegrated, the formation is not sufficiently well exposed for accurate measurement of its thickness. It is, moreover, overlapped by the Cretaceous strata. Probably the thickness of the beds exposed in Burnet County does not exceed 400 feet. The base of the formation is everywhere well defined, the change from the underlying limestone being abrupt. The formation comprises the latest Paleozoic rocks of the region, and the period of erosion indicated by the beveling of its beds corresponds to the great interval between Paleozoic and Cretaceous sedimentation. The formation is confined to the southeastern part of the Burnet quadrangle and to a small area on Riley Mountain in the Llano quadrangle."
The Marble Falls Limestone and this portion of the Smithwick Formation are both of lower Pennsylvanian age. In case you were wondering, the Pennsylvanian Epoch of the Carboniferous Period ranges from 318 to 299 million years old. During this time, the first reptiles evolved.
[more to come]
No comments:
Post a Comment