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Southern California Geology

Geologic Setting of the Transverse Ranges Province

San Bernardino Mountains

Upper Cenozoic Sedimentary Rocks

Isolated patches of upper Cenozoic sedimentary occur throughout the San Bernardino Mountains. The major outcrop belts occur in four main areas:

  • The Santa Ana Sandstone and related deposits
Sedimentary deposits assigned to the Santa Ana Sandstone (of Vaughan, 1922, as used by Sadler, 1982, 1993, and by Sadler and Demirer, 1986) crop out in an east-trending belt 5 km wide and about 30 km long on the south margin of the San Bernardino Mountains in the Santa Ana River drainage. The formation consists of several subunits that include poorly sorted sandstone, pebbly sandstone, conglomerate, reddish paleosols, and green claystone and mudstone (Strathouse, 1982a, 1983; Jacobs, 1982; Sadler and Demirer, 1986; Sadler, 1993). Some local sequences contain distinctive basement-clast populations that include Pelona Schist and anorthosite (Sadler and Demirer, 1986; Sadler, 1993). In the Barton Flats area, basalt flows and dikes interlayered with the sequence yielded a K/Ar whole-rock age of 6.2 Ma (Woodburne, 1975; Sadler, 1982b), but the sequence probably ranges from about 18 Ma to less than 5 Ma (Sadler, 1985, 1993).
Deposits similar to the Santa Ana Sandstone occur intermittently east of the main outcrop belt, and ultimately trend eastward toward sedimentary rocks that underlie extensive Upper Miocene basalt flows in the Pioneertown area of the eastern San Bernardino Mountains (Dibblee, 1967b). The Santa Ana Sandstone also trends west toward the Cajon Pass area where upper Miocene deposits of the Crowder Formation (as used by Meisling and Weldon, 1989) crop out. Meisling and Weldon (1989) view the Crowder and Santa Ana formations as part of a regionwide package of nonmarine upper Cenozoic sediment that originally blanketed much of the now-uplifted San Bernardino Mountains. Isolated patches of quartzite cobbles that occur locally throughout the San Bernardino Mountains (Sadler and Reeder, 1983) may be vestiges of this late Miocene blanket, along with faulted and folded sedimentary materials in the Big Bear Lake and Delamar Mountain areas (Matti and others, 1993).
  • Deposits of the nonmarine Old Woman Sandstone
The Old Woman Sandstone (of Shreve, 1968, as used by Sadler, 1981) crops out intermittently along the north front of the San Bernardino Mountains. The unit's lithology varies geographically (Sadler, 1981), but generally it consists of sandstone and mudstone interlayered with pebble- and cobble-bearing sandstone and conglomerate. The conglomerate beds contain basement-rock clasts derived from various source areas depending on their position within the formation (Powell and Matti, 1998a,b): (1) northern Mojave Desert sources shed sediment into the northern part of the Old Woman Basin throughout much of its history; (2) San Bernardino Mountains sources shed sediment into the southern part of the Old Woman Basin throughout much of its history, especially during the early Quaternary when the range was being uplifted rapidly to its current prominence. The unit ranges in age from possibly 6 Ma to perhaps 1.0 Ma (May and Repenning, 1982).
  • Deposits of nonmarine Mill Creek Formation

The Mill Creek Formation (of Gibson, 1964, 1971 as used by Matti and others, 1992b) crops out extensively in the vicinity of Yucaipa Ridge and lower Mill Creek between the Mission Creek and Wilson Creek strands of the San Andreas fault. The sedimentary formation consists of mudstone, sandstone, and conglomerate deposited in alluvial-fan, riverine, and lacustrine (lake) environments during the Miocene Epoch . Woodburne (1975) reviewed permissive evidence supporting a late Miocene age for the unit, but its age is not well established. Layered strata of the formation are beautifully exposed in roadcuts along State Route 38 in Mill Creek Canyon and in the canyon cliffs above the creek.

Steep slopes underlain by the Mill Creek Formation are prone to failure, particularly where mudrock is interlayered with sandstone and conglomerate, and the formation is marked by numerous small and large landslides. Youthful and older landslides are particularly prominent west and east of State Route 38 in Mill Creek Canyon and along the southwest base of Yucaipa Ridge east of the mouth of Mill Creek.

  • Deposits of the nonmarine Formation of Warm Springs Canyon

The Formation of Warm Springs Canyon is a name applied informally by Matti and others (1992b) to a poorly exposed sequence of nonmarine sandstone and conglomerate that crops out along the south margin of the San Bernardino Mountains from the Mill Creek Canyon area west to the Waterman Canyon area and beyond. The unit occupies the same structural position as unnamed sedimentary rocks mapped by Morton and Miller (1975, figs. 1c-1g) along the southwest margin of the San Bernardino Mountains. Gibson (1964, 1971) mapped this interval within his Mill Creek Formation (also see West, 1987), but Dibblee (1982b) referred to the rocks as Potato Sandstone and Hillenbrand (1990) as Potato Formation.

Sedimentary rocks of the Warm Springs Canyon formation crop out in a narrow belt between the Mill Creek and Wilson Creek strands of the San Andreas Fault. Its internal stratigraphy is broken and disrupted by faults confined to the formation's outcrop belt. No age has been determined for the rocks, but lithologic comparison with other nonmarine sedimentary rock sequences in the region suggests that it probably is late Miocene in age. The formation consists of heterogeneous sedimentary rocks dominated by sandstone, conglomerate, and conglomeratic sandstone interlayered with mudrock in some stratigraphic intervals.

The following text from Matti and others (1992b, p. 1-2) discusses uncertainties concerning how the Mill Creek and Warm Springs Canyon formations relate to each other spatially and geologically:

"Stratigraphic relations between the Mill Creek Formation and the Warm Springs Canyon formation east of the Wilson Creek fault are problematical, and are interpreted in different ways by different workers. Most investigators suggest a stratigraphic and paleogeographic link between the two units. For example, Dibblee (1982b, fig. 7) grouped the two units together within the Mill Creek Formation, and implied that they were deposited together elsewhere in southern California before being displaced jointly into the Mill Creek region by his north branch of the San Andreas fault (the Mill Creek strand of our usage). Demirer (1985), Sadler and Demirer (1986), and West (1987) elaborated this concept by proposing that the two sedimentary sequences are halves of a two-sided depositional basin that developed within a rift zone of the ancient San Andreas fault (also see Sadler and others, 1993). Despite the conclusions of these earlier studies, we maintain that sedimentary rocks we assign to the Mill Creek and Warm Springs formations are two distinct and different sequences separated by the Wilson Creek fault . We acknowledge lithologic similarities between them, but we emphasize three points that in our judgment warrant separation of the two units: (1) the Warm Springs Canyon formation is a sequence that overall is compositionally and texturally more immature than the Mill Creek Formation; (2) the geometry and pattern of sedimentary facies in the Mill Creek Formation is far more orderly than in the Warm Springs Canyon formation; and (3) we have not observed the two sequences to interfinger with each other as indicated by Demirer (1985) and West (1987).


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Selected References

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