Petrified Wood in Chinle Formation on Hurricane Mesa in southern Utah & views of Zion National Park скачать в хорошем качестве

Petrified Wood in Chinle Formation on Hurricane Mesa in southern Utah & views of Zion National Park

Searching for petrified wood in the Shinarump and Petrified Forest members of the Chinle Formation in Washington County, Utah. Directions: Drive on Utah Highway 9 to Virgin, Utah -about 20 miles west of Zion National Park- and head north on Mesa Road Park Car Here: 37.241879, -113.202591 Petrified Wood: 37.244303, -113.194201 Geologic Descriptions: Petrified Forest Member of Chinle Formation (237-201 million years old) Varicolored, typically gray to purple mudstone, claystone, and siltstone, lesser white to yellow-brown sandstone and pebbly sandstone, and minor chert and nodular limestone; regionally divisible into three parts, in ascending order: (1) the bentonitic Blue Mesa, (2) the pebbly sandstone of the Moss Back or Sonsela (depending on clast provenance), and (3) the bentonitic Painted Desert (Lucas, 1993; Woody [2006], however, suggested restricting the Petrified Forest to strata above the Sonsela, that is, to Lucas’ Painted Desert unit); Lucas (1993) proposed elevating the Chinle to group status, but that change has not been formally completed; the Owl Rock Member of the Chinle Formation may be present at the top of the unit as mapped here (Spencer Lucas, New Mexico Museum of Natural History and Science, verbal communication, August 2005), and the lowermost part of the Petrified Forest Member as mapped here locally contains light-gray, fine- to medium-grained sandstone with abundant petrified wood and cobble- to small boulder-size “cannonball” sandstone concretions that may represent a distal, unmapped facies of the Monitor Butte Formation; the middle part of the member is marked by a distinctive, thick-bedded, white to yellow-brown sandstone and pebbly sandstone with well-rounded chert and quartzite clasts likely correlative with the Sonsela; swelling, bentonitic mudstone and claystone are common throughout the member and although typically poorly exposed, their bright purple, grayish-red, dark-reddish-brown, light- greenish-gray, brownish-gray, olive-gray, and similar hues locally show through to the surface – they weather to a “popcorn” surface and are responsible for numerous building foundation problems in the region; typically poorly exposed and commonly forms landslides; contains petri ed wood, especially in sandstone beds; upper, unconformable contact is at the top of brightly colored swelling mudstones with nodular limestone, above which is a thin chert-pebble conglomerate and overlying reddish- brown, non-bentonitic siltstone and ne-grained sandstone; deposited in a variety of fluvial, floodplain, palustrine, and lacustrine environments that formed inland of a magmatic arc associated with a subduction zone along the west coast of North America (Stewart and others, 1972a; Dickinson and others, 1983; Lucas, 1993; Dubiel, 1994; DeCourten, 1998; Lucas and Tanner, 2007); about 400 to 650 feet (120– 200 m) thick. Shinarump Conglomerate Member of Chinle Formation (237-201 million years old) Medium- to coarse-grained sandstone, pebbly sandstone, and lesser pebbly conglomerate, locally with silty sandstone, claystone, and smectitic claystone interbeds, that forms prominent cliffs, hogbacks, and mesas; clasts are subrounded chert and quartzite; mostly thick bedded with both planar and low-angle cross-stratification, although thin, platy beds with ripple cross-stratification occur locally; predominantly pale- to dark-yellowish- orange, but pale-red, grayish-red, pale-orange, and pale- yellow-brown hues are common; commonly stained by iron-manganese oxides, locally forming “picture stone”; contains poorly preserved petri ed wood and plant debris, commonly replaced in part by iron-manganese oxides; upper contact marks a sharp topographic break and typically corresponds to the first appearance of varicolored, swelling mudstone; deposited in braided streams that flowed north and northwest (Stewart and others, 1972a; Dubiel, 1994; DeCourten, 1998); typically about 100 feet (30 m) thick, but ranges from 5 to 250 feet (2–75 m) thick due to deposition over paleotopography developed on the TR-3 unconformity. Source: Biek, R.F, P.D. Rowley, D.B. Hacker, J.M. Hayden et al., 2010, Geologic map of the St. George and East Part of the Clover Mountains Quadrangles, Wasington, and Iron Cos, UT: Utah Geological Survey Map 242DM. 32 / 2186831