Middle Tertiary Streams, The Beavertail Conglomerate and Related Units
Middle Tertiary stream deposits of central Arizona form the focus of this presentation. A number of these deposits extend across central Arizona and include the Beavertail conglomerate, Paulden formation, Cherry conglomerate and parts of the Chalk Canyon formation and Whitetail conglomerate (there is some debate about which, if any of these are formal stratigraphic units). They share several characteristics which include (1) coarse-grained (granules, pebbles, cobbles, and locally boulders), (2) lithic clasts of varying compositions derived from Paleozoic sedimentary and Proterozoic igneous and metamorphic rocks, (3) usually base of local Tertiary section, (4) rare to absent Tertiary volcanic clasts (pre-volcanic in local area), (5) clearly formed below an older Mogollon Rim, (6) general SE-directed paleoflow directions, (7) generally abruptly overlain by Middle to Late Miocene basalts. Taken as a whole, they indicate moderate paleorelief, the presence of the Mogollon Rim, and an integrated (though not fully reconstructed) and incised braided stream system.
The following diagrams and photos document the nature of Middle Tertiary braided stream deposits.
Time-stratigraphic correlation chart of Middle Tertiary conglomerates in central Arizona No basin-wide correlation of Middle Tertiary deposits has been accurately established. Age and correlation are based on stratigraphic position, age of included clasts, especially 23 - 28 Ma Sullivan Buttes Latite clasts, and topographic relations of conglomerate. The strike valley in which many of the conglomerates occur, had to be carved, substantial material removed, and a drainage network established before deposition was possible.
Detailed stratigraphic column of Beavertail conglomerate, Beavertail Butte The most complete and some of the best exposures of Middle Tertiary conglomerate occur in the Beavertail Butte area south of Sedona. Beavertail Buttes are vents and flows of Middle Miocene House Mountain Basalt (Hickey Basalt correlative) dated at 12 - 15 Ma (Ranney, 1988). The basalt directly overlie the Beavertail conglomerate. The conglomerate rests unconformably on Permian Hermit Formation and Schnebly Hill Formation; several hundred meters of local relief is obvious below the conglomerate. The lower unit of the conglomerate contains clasts derived exclusively from the nearby Mogollon Rim. This unit partially fills a deep paleocanyon cut obliquely into the Rim. The middle unit consists of mudstone, marlstone, limestone, and fine sandstone. The upper unit consists of conglomerate derived from Proterozoic and Lower Paleozoic rocks and represents widespread braided stream deposition in a strike valley parallel and below the Mogollon Rim.
Photographs that document Middle Tertiary stream deposition
Upper unit of Beavertail conglomerate at Beavertail Buttes. The middle unit is exposed at the bottom of the photo. Clasts are derived from Proterozoic and Lower Paleozoic rocks now exposed to the south and west. Some of these rocks were at least 50 km to the SW.
Upper unit of Beavertail conglomerate along Oak Creek at Hidden Valley, approximately 15 km west of Sedona. Here the lower and middle units are absent and the conglomerate rests directly on the Hermit Formation (Permian). Hidden Valley is where Oak Creek carves through western House Mountain.
Paulden formation overlain by Sullivan Buttes Latite at eastern Sullivan Buttes. The tan Paulden conglomerate outcrops in the slope above Granite Creek. Volcanic units form the overlying dark cliffs and ledges.
Upper unit of Beavertail conglomerate in paleocanyon SE of Sedona The light-colored conglomerate overlies the Permian Schnebly Hill Formation. The conglomerate fills a paleocanyon carved at least 800 m into the Permian rocks. Rim basalt dated at 6 - 8 Ma overlies the conglomerate and spilled into the paleocanyon from the Mogollon Rim (faint blue in background).
Late Miocene-Pliocene Verde Formation on Permian Schnebly Hill Formation, House Mountain. The Beavertail conglomerate at Hidden Valley (three photos above) is to the left and 200 m below the butte in this photo. The Schnebly Hill Formation formed the lower ramparts of the Mogollon Rim during deposition of the conglomerate. Millions of years later, the Verde Formation was deposited onto this rampart.
The following photo show some of the source rocks for Middle Tertiary conglomerate
Precambrian schist and quartzite (tan ridge), Bradshaw Mountains. The Bradshaws were probably high during much of the Tertiary and formed an important source for Tertiary conglomerate.
Close-up of Precambrian schist, Bradshaw Mountains. Paleogeographic reconstruction suggests that some schist fragments were transported at least 50 km during the Middle Tertiary, a long ways for such non-resistant clasts.
Proterozoic Del Rio Quartzite, Chino Valley. Though an extremely durable rock type, these clasts are rare in Tertiary conglomerates.
Proterozoic Red Rock Rhyolite, New River Mountains. Like the Bradshaws, these mountains were apparently high during much of the Tertiary. Rhyolite clasts are locally abundant in Tertiary conglomerates.
Cambrian Tapeats Sandstone nonconformably overlying Proterozoic schist, Chino Valley. Rare clasts of Tapeats Sandstone in Tertiary conglomerate are easy to identify in the field.
Devonian Martin Formation (dolomitic limestone) and Mississippian Redwall Limestone, Verde River near Chino Valley. Clasts of both these units are locally abundant and easily identifiable in Tertiary conglomerate.