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Geology - Cajon Pass
Cajon Pass: a classic geologic textbook
Robert E. Reynolds 1 and Michael O. Woodburne 21 CSU Desert Studies Research Associate
2 Department of Earth Sciences, Emeritus, University of California
Introduction
The San Andreas Fault Zone (SAFZ) in Cajon Pass marks the eastern margin of the Pacific Oceanic Plate and the western margin of the North American Continental Plate. At Blue Cut in Cajon Pass, the Pacific Plate consists of highly metamorphosed glaucophane and actinolite schist. Across the SAFZ to the east, the North American Plate consists of Mesozoic granitic rock, Cretaceous and Oligocene marine sediments, with a 20 million year history of continental basin filling, followed by the uplift of the Transverse Ranges. Students of life history can study changes in marine life and terrestrial mammals from investigate the biostratigraphic record spanning 70 million years.
Structure
Figure 1. Geologic map of the Cajon Pass area, adapted from Morton and Miller, 2008, showing major highways and faults. Qof: Old alluvial fan deposits; Qsh: Shoemaker Gravel; Qh: Harold Formation; Tcr: Crowder Formation; Tcv: Cajon Valley Formation units; Tv: Vaqueros Formation; Kcd: Cretaceous Cosy Dell Formation.
Cajon Valley (Figure 1) is now bounded on the west by the San Andreas Fault, which also cuts the Cajon Valley Fault that forms the western margin of the Cajon Valley Beds. During deposition, the proto-San Andreas Fault was forming in northern California at about 16 Ma (Nicholson et al., 1994). At this time the San Gabriel Fault was the major element of the proto-San Andreas system in the Cajon area, and it is likely that the Cajon Valley Fault was associated with the San Gabriel Fault from about 12 Ma (Woodburne, 2015). Subsequently, at about 6 Ma, the modern San Andreas Fault was developed in the Cajon Valley area, associated with the opening of the Gulf of California. The Squaw Peak thrust fault, which now separates the Crowder Formation and Cajon Valley Beds, was active from about 9 Ma (Meisling and Weldon, 1989), and led to the modern close proximity of those sedimentary units.
About 18 million years ago, the activity on the proto-San Andreas system caused clockwise rotation of regions that later became San Gabriel Mountain Range and San Bernardino Mountain Ranges (Dickinson, 1996). Rotation opened basins that were filled by the Cajon abstract—In Cajon Pass, students can take a short walk across the San Andreas Fault Zone from the Pacific Oceanic Plate to the North American Continental Plate and investigate depositional environments that contain a biostratigraphic record that spans 70 million years. Fossils provide the clues.
An articulated elasmosaur verifies a late Cretaceous age for the marine Cosy Dell Formation. The younger, earliest Miocene Vaqueros Formation contains mollusks, rays, and the dolphin Allodelphis woodburnei. These marine strata are overlain by deposits of continental arkose derived from the central Mojave Block. Fossil plants and gastropods in the Cajon Valley Beds are associated with bones and teeth of mammals that span the early Miocene Hemingfordian and Barstovian North American Land Mammal Ages (NALMA). Adjacent to the east, across the Squaw Peak Fault, is the Crowder Formation. Deposition of the Crowder sediments started during the Hemingfordian NALMA, as did the Cajon Valley Beds, but mammal fossils suggest that deposition of the first half of the Crowder continued through 7 Ma in the late Miocene. Projecting increased rates of deposition for the upper half of the section, the Crowder Basin may have been receiving sediment from the Victorville area until 4 Ma.
Although both Miocene basins are presently in proximity, it appears that the Crowder basin was located farther east than today. As noted above its depositional history was different from that of the Cajon basin. In addition to their Mojave source, late Barstovian elements of the northwestern Cajon Valley beds were in part derived across elements of the then-active proto-San Andreas fault.
More recent activity on the San Andreas Fault accompanied the continued rising of the San Gabriel Mountains, depositing Pleistocene sediments near the top of the In-face Bluffs. These units include the Phelan Peak Formation which records the change in drainage at 2 Ma, from south toward the Pacific Ocean to north into the internally drained Mojave Block. The Phelan Peak is overlain by the Harold Formation, Shoemaker Gravels and Noble’s Old Alluvium, the latter recording the Brunhes–Matuyama reversal (781,000 years ago). The 70 Ma long biostratigraphic sequence of Cajon Pass has been exposed by northerly headward erosion along the drainages of Cajon and Crowder creeks.
Valley Beds (Woodburne and Golz, 1972) from 18 to 12.7 Ma (Liu, 1990) and the Crowder Formation from 18 to about 3 Ma (Weldon, 1985; Liu, 1990; Reynolds and others, 2008). The Miocene basins were covered with north-flowing Pliocene and Pleistocene debris when the eastern Transverse Ranges started rising about three million years ago. The Phelan Peak Formation is overlain by the Harold Formation, Shoemaker Gravels and Noble’s Old Alluvium, the latter recording the Brunhes– Matuyama reversal (781,000 years ago), near the top of the In-face Bluffs. Those ranges reached their maximum height (11,000 ft) only a half a million years ago.
Stratigraphy and biostratigraphy
Cretaceous Cosy Dell Formation
The oldest nonmetamorphosed sediment in Cajon Pass is the Late Cretaceous marine San Francisquito Formation (Kooser, 1985; Lucas and Reynolds, 1991). The “Paleocene” San Francisquito Formation was renamed as the Cretaceous Cosy Dell Formation (Morton and Miller, 2008) due to the presence of forty articulated elasmosaurid plesiosaur vertebrae (Lucas and Reynolds, 1991), with elasmosaur being a late Cretaceous indicator fossil. Other associated fossils include crustacean pinchers, fish scales, scaphopods, and gastropods. Deposition of the Cretaceous Cosy Dell Formation was followed by a local 40 Ma depositional hiatus. Comparisons of other San Francisquito Formation outcrops could determine if later sediments have also been removed by an erosional event of unspecified cause.
Early Miocene Vaqueros Formation
The early Miocene Vaqueros Formation contains the long-snouted dolphin Allodelphis woodburnei of the superfamily Platanistoidea (Barnes and Reynolds, 2007, 2008), an early marine relative of the fresh water Ganges River dolphin. The Vaqueros Formation is rich in oyster shells and barnacle fragments. The dolphin skeleton was associated with fossils of elasmobranchs, the echinoid Scutella fairbanksi, and the mollusks Crassatella granti, Ostrea titan subtitan, and Pecten sespeensis. The presence of the gastropod mollusk Turritella inezana in this marine deposit helps to establish its Early Miocene age (22Ma, Woodring, 1942).
Middle Miocene Cajon Valley Beds
The Cajon Valley Beds (Units 2 – 6) contain terrestrial land mammals that represent the late Hemingfordian through Barstovian NALMA (Woodburne and Golz, 1972; Reynolds et al., 2008). Magnetostratigraphy (Liu, 1990) shows that deposition of Units 2–6 continued for five million years from 17 Ma until 12.7 Ma. Fossil mammals include the oreodont (Woodburne and Golz, 1972), peccary, small and large camels, deerlets, small forest horses, large grassland horses, rhinoceros, chalicotheres (Coombs and Reynolds, 2015), rabbits and rodents, hedgehogs, shrews, badgers and bears (Wagner and Reynolds, 1983; Reynolds, 2015).
Miocene Crowder Formation
Magnetostratigraphy (Weldon, 1985; Liu, 1990) indicates that deposition of the Crowder Fm began 17+ Ma. Time-diagnostic rodents at the top of Unit 3 in the middle of the Crowder sequence indicate a Hemphillian age of 7.1 Ma (Reynolds and others, 2008). The greater than twelve million year depositional record has produced Hemingfordian, Barstovian and Hemphillian millipedes, lizards, small and large camel, antelope, forest horses, grassland horses, tapir, rhinoceros, weasels and badgers (Lofgren and Abersek, 2018), bone-crushing dogs, rabbits and rodents (Lindsay and Reynolds, 2008), squirrels and flying squirrels, shrews and hedgehogs (Reynolds and others, 2008).
Although presently separated from the Cajon Valley Beds by the Squaw Peak Fault, the Crowder basin was originally located farther east and its depositional history differs from that in the Cajon basin. Both formations have a source of lithic clasts from the Mojave Block. The late Barstovian elements of the northwestern Cajon Valley beds were in part derived from the northwest across elements of the then active proto-San Andreas Fault. Current directions within the lower and upper Crowder Formation developed by Foster (1980) indicate most sources for clasts in the Crowder were from the north. Projections of depositional rates (Reynolds and others, 2008) suggest that the upper Crowder Formation unit 5 may be younger than 5 Ma, and within the 4-3 Ma time range of the south flowing ancestral Mojave River (Cox and others, 1998; Cox and Tinsley, 1999; Reynolds and Cox, 1999; Cox and others, 2003).
Phelan Peak Formation
Magnetostratigraphy and clast analysis (Weldon, 1985) of the Phelan Peak Formation demonstrates that the lower unit was receiving sediment at 4 Ma from the Mojave Block to the northeast. The middle unit consists of playa silts and pond sediments deposited in a west northwesterly trending basin parallel to the SAFZ. The upper unit started receiving clasts from the San Gabriel Mountains to the west prior to 2 Ma (Meisling and Weldon, 1989). The Phelan Peak Formation is overlain unconformably by time- and topography-transgressive Pleistocene sediments derived from the eastern Transverse Ranges to the west and south. These units are exposed in the In-face Bluffs around the northern margin of Cajon Valley. Harold Formation
In Cajon Pass, the Harold Formation was deposited unconformably upon the Phelan Peak Formation between 1.7 and 1.4 Ma (Weldon, 1985). The Harold Formation consists of arkosic conglomerate with occasional paleosols.
Shoemaker Gravels
The Shoemaker Gravels were deposited conformably upon the Harold Formation between 1.4 and 0.8 Ma (Weldon, 1985), and are coarse arkosic sediments with coarse clasts of varying size.
Nobles Old Alluvium
The Old Alluvium of Noble contains clasts coarser than the Shoemaker Gravels, signifying an increase in rate of uplift of the San Gabriel Mountains. The Old Alluvium was shed from the San Gabriel Mountains starting at 0.8 through 0.4 Ma and contains the Brunhes/Metayama reversal of 781,000 years (Weldon, 1985). The coarseness shows that the San Gabriel Mountains reached their maximum height only 500,000 years ago.
Victorville Fan Complex
Together, the Harold, Shoemaker and Old Alluvium make up the Victorville Fan Complex. This sequence records drainage from the Mojave Block southwest toward the Pacific Ocean and subsequent reversal of drainage northeast into the Mojave Block as the Transverse Ranges began to rise. At Victorville, deep borehole data demonstrate that south-trending streams brought clasts of Jurassic and early Miocene volcanic rocks from the Mojave Desert, not from the Transverse Ranges, and include lithologies from the Kramer Hills and outcrops near Barstow. Borehole magnetostratigraphy suggests that the top of the south-flowing lower unit is about 2.5 Ma (Cox and others, 1998). This would be within the 4 to 2 Ma range of the lower unit of the Phelan Peak Formation in western Cajon Pass, or the projected age range of the Crowder Formation (4-3 Ma, Reynolds and others, 2008) in eastern Cajon Pass.
Present Cajon Basin
Headward erosion of the Cajon Creek drainages followed the brecciated rock of the SAFZ. When encountering the softer sediments on the North American Plate, the drainage system widened to include West Cajon Wash and Crowder Creek. Broadening of the drainages systems in the Cajon basin created an amphitheater bounded on the northwest and northeast by the In-face Bluffs. Many questions remain in Cajon sediments to be answered by students of geology and paleontology. A few of those questions include:
A) Does the fossil record in the Cretaceous Cosy Dell Formation suggest the presence of the Cretaceous/ Tertiary Boundary? Can the basal conglomerate describe the original geographic position of this formation? What was the cause of the overlying 40 Ma erosional hiatus?
B) What were the structural events that caused the many different sedimentary facies and inferred habitats in the Cajon Valley and Crowder formations?
C) Do clasts and facies in the upper Crowder Formation suggest a connection with south-flowing sedimentary sources from Victorville between 4 to 3 Ma?
Acknowledgements
The authors thank David Miller for thoughtful review and comments that benefited this paper. The authors also acknowledge the many hours assistance and research contributed by students and museum volunteers that produced the current biostratigraphic picture that we have of the sequence in Cajon Pass.
References
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Kassie picking at North American Plate while standing on Pacific Plate
View south, down Cajon Canyon from Vaqueros Formation