South and East of the Petrified forest east of Holbrook, this limited extent unit was named by Cooley in 1958 for the excellent exposures near the base of the type locality at Mesa Redondo 35 miles south east of Holbrook. The unit consists of a mudstone and siltstone facies separated by a conglomeritic sandstone unit lies between the Shinarump and the Blue Mesa member or the Monitor Butte member of the Chinle. Most of the unit is composed of pink, purple or dusky red sandstones formed from channel deposits and contain low angle crossbedding. Mesa "Redondo" and "Redonda" are the same butte and the names are used synonymously. To add to the confusion, there is a Redonda Formation, near Mesa Redonda in Eastern New Mexico that is also Triassic and in the Chinle Group! This was terribly confusing to us for a long time in that the literature doesn't usually specifiy which Redonda they are referring to.
Mesa Redondo, the locality which the formation is named after for the Mesa Redondo member of the Chinle Group. Located South of Holbrook, this volcanic topped mesa has a base of pastel grays formed from the Chinle (bottom of photo), overlain by the basal Dakota formation (Cliff Dwellers Member), and the bright yellow unit is the Twowells sandstone tongue of the Dakota Formation, which is Cretaceous in age.
Purple and Maroon bentonites and shales of the Mesa Redondo Member form badlands and cone shaped mounds. The yellow overlaying cap is Cliff Dwellers member of the Dakota formation (Cretaceous) Although we found no vertebrate fossils here, the Chinle here did contain some excellent silicified roots, and some bland looking petrified wood.
The pastel colors of the Chinle at Mesa Redondo can be seen here clearly when the sun angle is right. I'm standing on the Basal Dakota, with its cherty conglomerate filled with pebbles of both Permian and Triassic cherts. This must not be confused with the more brown to white Sonsela Sandstone which divides the upper and lower Petrified Forest members.
Lacustrine mudstones, brown and gray tuffaceous bentonites and large blocks of basal Dakota mark the periphery of Mesa Redondo. We spent many hours exploring the almost alien landscape, searching for clues of its exciting past.
Large blocks of sandstone break off from the overlying cap of this ridge, and tumble down the wash. Such scenes are very common in the Chinle, and represent the deposition of a sand bar next to a stream bed over volcanic tuffaceous deposits. The sandstone can be very coarse, indicating rapid water movement during deposition.
One of the most amusing finds in the Mesa Redondo area in the Chinle was the beds of reddish and yellow cherts. Many of the 3 to 6 inch beds were laminated with fine detail very much like one would expect for an algal deposit. These fossil algal mats were probably a near shore event, growing in the shallows of a quiet lake.
The Author sits next to a large permineralized log in the Mesa Redondo Member. This preservation includes cellular detail and very fine trunk structure. Although not colorful like the agateized logs in the Petrified Forest, we find the perminerlized logs far more scientifically interesting.
This is why you don't want to take the family car out for a spin after a rain in the Chinle formation bentonites! The resulting mud is so sticky, your feet grow to two feet in diameter when you try to walk through it. Of course, once we got to higher ground, the situation was more walkable!
Along Highway 180 near Saint Johns, excellent exposures of the red sandstone unit of the Mesa Redondo formation can be found. Although no fossils were found, tabular bedding with some crossbed sets were observed.
Along county road 5392 off of Highway 180 near Saint Johns, the official type locality for the MR formation can be found. Here, sandstones and shales rest on a lower unit of the Shinarump formation. I am standing on an adjacent ridge, where pieces of petrified wood were found.
At the type locality above, another view of the reddish sandstones, orange and gray shales forming the Mesa Redondo formation. The sandstones to my right are very thinly bedded, but coarse and may represent a fluvial river side deposit. Some silicified wood was amongst the sandstone plates, but probably originated from an overlying bed that had eroded away.
Dawn explores the redbeds of the MR formation at the type locality. The ridge was mostly planar bedded on this side, and contained few sedimentary structures or fossils. The crumbly nature of the sediments made footing poor, and exploration sometimes too exciting.
The sandy shales on the other side of the ridge at the MR type locality were much more interesting. Beautiful sets of fluvial crossbeds marked with light yellow banding marked the individual elements clearly.
Close up of the above cliff face at the MR type locality showing the tan siltstones defining the crossbed sets in the red shales. Fluvial silt deposition may indicate a relatively active period of floodplain deposition. The bed was only about 10 feet thick.
At least six sets of crossbeds can be seen in this three foot area on the cliff face at the above locality. No fossils were found in the siltstones on this ridge.