THE GABLE CREEK FORMATION CHARACTERISTICS
AND CONDITIONS OF DEPOSITION
The Gable Creek Formation was named by Wilkinson and Oles, (1968), and mapped by Oles and Enlows, (1971) to represent a series of marine conglomerate layers exposed near Mitchell, Oregon.
They assigned a mid-cretaceous age to these rocks, based upon age determinations of fossils (chiefly ammonites) from mudstone beds between the conglomerates.
Individual layers vary in thickness from 10 to 600 feet. The more extensive conglomerates were mapped as numbered sheets (1 to 11) from base to top, although no single location was found in which all sheets were exposed together.
Most of the conglomerates are clast-supported, in that only granules and sand without mud occupy space between the clasts.
Size of the clasts varies from a few inches to several feet within single layers.
Grading of the clasts, both normal and reverse or from normal to reverse in a single sheet is common.
In some layers the grading continues upward until a pebbly sandstone grades into a pebbly mudstone.
The clasts are imbricated in some exposures and two-dimensionally oriented in nearly all layers, permitting determination of original horizontal configuration even after large-scale deformation.
Clasts that have been fractured and subsequently healed are found in many conglomerates that have been subjected to deformation.
All of the conglomerates are polymictic with many varieties of intrusive and extrusive igneous rocks. Granites and granodiorites with high-grade to low-grade metamorphic rocks and clasts of fossiliferous sandstone are common.
Volcanic rocks predominate, with porphyritic to aphyric rhyolites in the form of lavas and welded ignimbrites being the most common.
Some conglomerate layers can be distinguished by their clast lithologies. But a conventional census of plutonic, volcanic, metamorphic, etc. clasts is not successful. However, very distinctive and unusual lithologic types that are easy to recognize in the field are restricted to some units. Good examples are the very coarse red granites with unusually large quartz grains, rhyolites with phenocrysts of both K-spar and quartz, and densely welded ignimbrite clasts. Where this has been accomplished, several of the conglomerate layers seem not to represent broad sheets but to have been deposited in channels that trend from northeast to southwest. It is significant that conglomerate beds become thicker and mudstone beds become fewer in the northeast part of the inlyer while the opposite is true to the southwest. Pertinent to this, on the east limb of the Mitchell Anticline a distinctive bed of marine volcaniclastic rock composed of ash and pumice serves as a marker horizon within the Gable Creek conglomerates.
In this bed, pumice fragments become coarser (up to one-half inch) toward their northeast outcrops. This strengthens speculation that Gable Creek conglomerates, containing dominant volcanic rocks, originated from a northeastern provenance of active volcanism.
The cretaceous marine rocks of central Oregon present a puzzling contrast. What conditions could be responsible for long-continued deposition of uniform muds over such an extensive part of the Cretaceous seafloor, interrupted by energetic floods of widespread coarse conglomerates? This concern was expressed by Oles (Oles K.F. and Enlows H. E., 1971) who suggested that a near-shore fluvial-deltaic system was frequently invaded by floods from “swinging distributaries of one or more great rivers” and that “each Gable Creek tongue was succeeded by a marine tongue of the Hudspeth Formation, representative of a renewed transgression by the sea.” This interpretation was questioned by Kleinhans (Kleinhans, L. C., Balcells-Baldwin, E. A., and Jones, R. C., 1984) who suggested that both Gable Creek and Hudspeth sediments were not of fluvial-deltaic origin but were deposited within a submarine fan environment. Emphasis was placed upon the common occurrence of turbidite sands in the Hudspeth units and a lack of shallow-water features such as mud cracks and weathering as had been reported by Oles.
My own observations of turbidite sandstones so common in Hudspeth sections and my recognition of exceptionally energetic processes of Gable Creek deposition cause me to suggest another possible origin of these sediments. The Gable Creek layers nearly all exibit graded bedding which is widely regarded as characteristic of turbid conditions of deposition.
Similarly, rip-ups of mudstone fragments which are so common in Hudspeth sandstones are also present in Gable Creek conglomerates.
Enormous rip-ups of angular Hudspeth mudstone, often as large as 10 feet are enclosed at the base of some Gable Creek layers. Flute casts which are so common in Hudspeth sandstones, also occur at the base of some Gable Creek layers but were formed with conglomerate clasts instead of sand grains.
Some of them present a tapered spiral (corkscrew) configuration. The very large size of some Gable Creek boulders (up to two feet in diameter) must require strong currents to carry them over the seafloor.
It should be considered that “ordinary” conglomerates, mixed with a normal component of finer debris, might have been deposited beyond a fluvial-deltaic environment to the edge of the continental slope. The coarser components might accumulate multiple times as finer material is carried farther down slope. If the coarse-accumulate becomes unstable, and moves collectively to deeper levels, it might produce a “resedimented” conglomerate of clast support and many features of turbid deposition.