SHINARUMP FORMATION (Shinarump Conglomerate)



(Uploaded 1/3/09)

The basal Chinle is the Shinarump Formation, and represents channel deposits from a fast moving waterway. Silicaceous gravels derived from the underlying Kaibab formation contain Paleozoic fossils such as bryozoans and brachiopods of Permian age. We also find bits of petrified wood from the Triassic period incorporated in the conglomerates from the underlying Moenkopi formation. The Shinarump consists of firmly cemented sandstone, mud stone and conglomerate beds that are lenticularly cross bedded units. The conglomerate unit is composed of well rounded to sub angular pebbles and cobbles of quartz, quartzite, jasper, chert and petrified wood. The Shunarump formation is lenticularly bedded and consists of a maze of thin to thick interlaced channel deposits. the sandstone and conglomerate units commonly have trough cross bedding, and the small to medium scale cross beds dip at low and medium angles. Small to large scale cross beds dipping at low and very low angles are formed in some of the sandstone beds,. Much of the mud stone has small scale cross beds visible in fresh exposures, however some of the mud stone units are thin and horizontally bedded. Cuspate ripple marks and ripple laminations are common in the sandstone beds. A similar unit, the basal Dakota Sandstone, the Cliff Dwellers Member is very similar even down to containing Paleozoic clasts from the Kaibab formation and must not be confused. The nominal thickness of the Shinarump in Navajo county is 25 to 100 feet.

Down county road 5392 near St. Johns, we find an excellent outcrop of the Shinarump formation. Here, large blocks of conglomeritic sandstone break off the top of a bed on the top of a ridge, and car sized blocks roll downhill.

Close up of one of the boulders, which is about 2m tall from the road 5392 outcrop. The cross bedding is clearly seen, and note how the ground is covered with hordes of small pebbles of the eroded conglomerate. The dark colored cherts consist of cherts and other volcanic material derived from earlier formations.

Superb river channel cross beds are preserved in this 2m wide block from the same site.

Large car sized blocks at the base of the outcrop at the same site. Low angle cross beds are the predominant sedimentary structure seen here. the cherts are embedded in a very coarse sandstone, indications of a high energy environment.

Close up of the Shinarump showing coarse texture and the scattering of larger pebbles of cherts, quartzites and basalts.

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