AI collection: to be reviewed
Introduction
Rainbow Basin near Barstow, California, exposes a thick Miocene sedimentary sequence folded into a broad syncline and carved into colorful badlands. The rocks record rivers, lakes, volcanic ash falls, and later deformation. This guide covers stratigraphy, tectonics, paleontology, mineralogy, geomorphology, and recent findings, with practical field pointers.
Stratigraphy (oldest to youngest)
Jackhammer Formation (early Miocene, possibly latest Oligocene–early Miocene)
- Thickness: about 200–300 m
- Lithology: conglomerate, arkosic sandstone, tuff and tuff-breccia, local basalt flows and dikes
- Environment: early basin alluvial fans and braided streams with nearby volcanism
- Field markers: dark basalt outcrops, volcanic breccias, rest nonconformably on quartz monzonite basement; limited exposures toward the north and Mud Hills
Pickhandle Formation (early Miocene)
- Thickness: about 1,000–1,400 m
- Lithology: gray conglomerates and arkosic to tuffaceous sandstones; andesitic and rhyolitic tuffs and tuff-breccias; very coarse granitic and rhyolitic megabreccia near top
- Environment: alluvial fans and debris flows interbedded with volcanic ash and vent-proximal deposits
- Field markers: massive gray slopes with abundant volcanic clasts; bouldery upper beds; forms higher, less colorful ridges around basin margins
Barstow Formation (middle to late Miocene)
- Thickness: about 800–1,100 m
- Lithology: basal conglomerates and sandstones grading upward to siltstone, mudstone, shale, and thin limestone; multiple air-fall ash beds including a conspicuous white marker tuff near the top
- Environment: river and lake deposits, evolving from more lacustrine to more fluvial upward.
- Field markers: multicolored banded slopes (greens, buffs, reds, whites); white marker tuff forms a traceable pale band; principal fossil-bearing horizons
Quaternary fanglomerate (late Pleistocene)
- Thickness: 10–30 m, variable
- Lithology: unconsolidated to weakly cemented alluvial fan gravels and sands
- Field markers: relatively flat-lying caps forming benches on ridge tops; angular unconformity over tilted Miocene beds
How to identify formations quickly in the field
- Variegated soft mudstones, prominent white ash bands, and most fossils indicate the Barstow Formation.
- Massive gray, bouldery, and volcanic-rich conglomerates indicate the Pickhandle Formation on basin flanks.
- Dark basalt and mixed volcanic breccias low in the section indicate the Jackhammer Formation.
- Flat gravel caps over tilted beds indicate late Pleistocene fanglomerate.
Tectonics and structure
- Basin formation began under extension, creating a subsiding trough that trapped Miocene sediments and ash.
- Post-depositional compression folded the section into the Barstow Syncline, a broad U-shaped downfold trending roughly east-west across Rainbow Basin.
- Later right-lateral shear related to the Eastern California Shear Zone produced northwest-trending faults and minor offsets within the basin, while nearby regional faults accommodated larger motions.
- Field expression: drive the one-way loop to cross from the north limb into the synclinal trough and up the south limb; watch bedding dip angles reverse. Look for the white tuff band bending across hillsides. Note flat Pleistocene gravels resting on steeply tilted Miocene strata along ridge tops (angular unconformity). Small lateral offsets in ash layers are visible in canyon walls.
Paleontology (practical overview)
- The Barstow Formation is the type area for the Barstovian land mammal age. Fossils include three-toed horses, camels, pronghorn relatives, oreodonts, small camelids, mastodons and gomphotheres, small rhinos, canids, felids, bear-dogs, tortoises, rodents, and water birds like flamingos.
- Fossil horizons are most common in the middle to upper Barstow Formation, often in carbonate-rich mudstones or channel sands.
- Trace fossils include camelid footprints preserved on bedding surfaces.
- Exceptional insect and arthropod fossils occur in calcareous lacustrine nodules within parts of the lower to middle Barstow Formation (more common toward the Calico side).
- Field etiquette: do not collect. Photograph in place, note locations, and report significant finds to the managing authorities or a museum.
Mineralogy and easy field clues
- Quartz and feldspar dominate sandstones and pebbly conglomerates (granite-derived). Look for sparkling quartz and blocky white or pink feldspar.
- Volcanic fragments and phenocrysts in Pickhandle and Jackhammer units include andesite and rhyolite pieces; tuffs may show altered glass shards now clay.
- Calcite occurs as thin limestone beds, cements, and white fracture veins; it may fizz in dilute acid.
- Bentonite and other smectite clays form from altered ash; soft greenish or light-gray layers can be slick when wet.
- Iron oxides stain red, orange, or yellow beds; greenish hues often reflect reduced conditions or clay composition.
- Zeolites (notably clinoptilolite) formed during alteration of ash in some tuffs; pale, light, porous textures are clues.
- Barite and other vein minerals may occur locally; the broader region hosts silver and borate deposits in Miocene lake beds east of the basin.
Geomorphology and what you are seeing
- Badlands topography forms as soft tuffaceous mudstones and shales differentially erode beneath slightly harder ledges, producing close-spaced gullies, ribs, and small hoodoos.
- Canyons such as Fossil Canyon (loop road), Owl Canyon, Snake Canyon, and Coon Canyon follow rock contrasts and fracture zones; short narrows can form where a resistant ledge crosses a wash.
- Terraced slopes appear where resistant ash or limestone beds overlie softer mudstones. The white marker tuff may create subtle benches.
- Gravel-capped mesas and ridges preserve remnants of late Pleistocene fans above a planed surface, recording uplift and renewed incision.
- Drainage is endorheic. Washes deliver sediment to nearby playas rather than to the sea.
Field itinerary (practical stops)
- Loop road pullouts: use them to scan the syncline. Trace the white marker tuff across both limbs. Note reversal of bedding dips as you cross the trough.
- Fossil Canyon floor: examine float (do not collect). Look for ash-rich, clayey horizons and carbonate nodules.
- Owl Canyon hike: walk upstream from the campground to observe upward transition from coarser channel beds to finer lacustrine intervals and scattered ash beds; look for small ledges and short narrows where resistant beds cross the wash.
- Ridge viewpoints: from safe, established paths, compare colors and textures across bands; identify Barstow vs Pickhandle by color, clast content, and resistance.
Recent findings and updates (practical takeaways)
- High-precision dating of ash beds refines Barstow Formation ages and ties local sequences to the mid-Miocene climatic optimum. Expect upper Barstow marker tuffs near about the mid to late Barstow interval.
- Facies mapping clarifies lateral changes: central Rainbow Basin records mixed fluvial–lacustrine settings, grading to coarser alluvial fans toward the Calico and Barstow margins. Expect more fossils and ash in the central to eastern fine-grained intervals; expect coarser fan deposits toward basin edges.
- Paleomagnetic and structural syntheses support modest clockwise block rotation since the Miocene, consistent with regional right-lateral shear. This helps explain present fold and fault trends.
- Paleoenvironment proxies (soil carbonates, phytoliths, tooth enamel isotopes) suggest a shift from more humid woodland to more arid, seasonal grassland-savanna through Barstow time, with periodic wildfire indicated by charcoal layers.
- Economic geology interest in lithium-bearing clays within altered ash-rich lacustrine beds highlights the geochemical richness of the Barstow system, though Rainbow Basin is a protected area.
Safety and stewardship
- Heat, sun, and flash floods are real hazards. Carry ample water, wear sun protection, and avoid narrow canyons if rain threatens.
- The badlands are fragile. Stay in washes or on existing paths to minimize erosion.
- Do not collect fossils, artifacts, or rocks from protected areas. Photograph and share observations instead.
Quick identification checklist
- Multicolored soft mudstones with white ash bands and fossil occurrences: Barstow Formation.
- Massive gray, bouldery, volcanic-rich slopes: Pickhandle Formation.
- Dark basalt and volcanic breccia low in section near basement: Jackhammer Formation.
- Flat gravel benches capping tilted strata: late Pleistocene fanglomerate.
Summary timeline
- Late Jurassic to Cretaceous: emplacement of quartz monzonite basement.
- Early Miocene: subsidence and deposition of Jackhammer Formation with local basalt.
- Early Miocene: deposition of thick volcaniclastic Pickhandle Formation during active volcanism and fan building.
- Middle to late Miocene: deposition of the Barstow Formation in lakes and rivers with multiple ash falls and rich fossil assemblages.
- Post-Miocene: folding into the Barstow Syncline; later right-lateral faulting and minor block rotation.
- Late Pleistocene to present: planation, fan deposition, incision, and sculpting of badlands.