| Popis: |
The Upper Jurassic La Plata sandstone, described by Cross as essentially two thick sandstones with a thin limestone between, is shown to consist typically of the following units in ascending order. Unit 1 is the Entrada sandstone, a massive, partly cross-bedded, yellowish to red sandstone. Unit 2 is discontinuous bituminous, thin-bedded limestone, in places associated with gypsum, attaining a thickness of 15 feet, and named the Pony Express limestone member of the Morrison formation. Unit 3 is sandstone or sandy beds about 20 feet thick here named the Bilk Creek sandstone member of the Morrison. At the top of this member there is a sandstone about 1½-2 feet thick, with coarse rounded grains and autochthonous red chert, called carnelian sandstone. Unit 4 consists of 5 -100 feet of red, sandy, cherty marls, named the Wanakah marl member of the Morrison, a restriction of the name to only that part of the section to which it was originally applied. The Wanakah marl contains crystals or concretions of barite and a zone of chert concretions coated with a green mineral. This zone is a valuable diagnostic bed in correlation. Unit 5 is massive, cross-bedded, whitish sandstone attaining a thickness of 500 feet, here named Junction Creek sandstone member of the Morrison. In places beds of Wanakah type overlie the Junction Creek sandstone and make it more difficult to draw, the boundary against the overlying beds. Unit 1, the Entrada, is Cross' lower La Plata sandstone. Unit 5 is thickest in the La Plata Mountains and is the upper La Plata sandstone of that region. In the Telluride region, where this bed is poorly developed or absent, the Bilk Creek sandstone (unit 3) has been called upper La Plata sandstone. In other areas, where both the Pony Express limestone (unit 2) and the Junction Creek sandstone (unit 5) are absent or poorly developed, the Bilk Creek sandstone has been regarded as part of the Entrada, and beds probably younger than the Junction Creek have been called upper La Plata sandstone. We believe that units 2 and 3 are equivalent to the Curtis of east-central Utah, and unit 4 to the Summerville, and that unit 5, the Junction Creek sandstone, hitherto recognized only as the upper a Plata sandstone of the La Plata Mountains, bears more resemblance to the Wingate, Navajo, and Entrada sandstones of the Utah section than to the overlying Morrison sandstones. Pending further studies in this area they are, however, all classed here as members of the Morrison formation. In southeastern Utah the Bluff sandstone member of the Morrison may be equivalent to the Junction Creek. We incline to Gregory's interpretation that the Todilto limestone in Todilto Park is overlain by the Navajo sandstone and is, therefore, not equivalent to the Pony Express limestone; but on Beclabito (Biltabito) Dome in the northwest corner of New Mexico, the limestone called Todilto lies on the Entrada, is overlain by beds resembling the Wanakah, and we, therefore, consider it equivalent to the Pony Express. Possible equivalents of the Junction Creek sandstone in New Mexico are pointed out. A tentative correlation of a section along the Cimarron River, in the northeast corner of New Mexico, with the southwest Colorado section, is proposed. The presence, in places, of a series of y llowish sandy shales and regularly bedded, sideritic sandstones, that may be intermediate in position between the horizon of the top of the Junction Creek and the overlying Morrison sandstones, is pointed out. |
