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Beds of the Glenns Ferry Formation (Pliocene to early Pleistocene) form the Chalk Hills about 23 km (14 mi) west of Nampa, Idaho, near the site of abandoned Froman Ferry on the Snake River. With one noteworthy exception, the beds there contain vertebrate fossils typical of late Blancan (Blancan V; ~2.6 Ma ~1.9 Ma) land mammal age assemblages found elsewhere in the formation. The noteworthy exception is the co-occurrence ofPhenacomys gryci , an immigrant microtine rodent and indicator species for Irvingtonian time (~1.9 Ma 0.9 Ma) in the coterminous United States. Paleomagnetic samples were collected from twelve horizons in an attempt to refine age control by identifying the Olduvai Normal Polarity Subchron (~1.87 Ma ~1.67 Ma) which is associated with the Blancan-Irvingtonian boundary. Sampled horizons were sufficiently close that, if present, an Olduvai zone could reasonably be expected to be identified at two or three of the horizons. Paleomagnetic samples from 11 of the 12 horizons are clearly of stable, reversed paleomagnetic polarity. Although characteristic reversed polarity directions could not be isolated by alternating field demagnetization from samples of the lowest horizon, changes in direction and intensity of magnetization during treatment indicate that they carry weak, but stable components of reversed polarity that are obscured by present field normal polarity overprints. Thus, we interpret all sampled horizons to be of primary reversed polarity, and do not recognize an Olduvai Normal Polarity Subchronozone in the section. If the section spans any part of Olduvai time, the zone is anomalously thin or missing. Introduction A paleomagnetic study of an 82 m-thick (270-ft) section of the Glenns Ferry Formation of Pliocene and early Pleistocene age was completed near the site of the now abandoned Froman Ferry (spelled "Frohman" on U.S.G.S. topographic maps and in some reports) on the Snake River in southwestern Idaho. The section was measured, sampled, and briefly described. The purpose of the study was to develop a paleomagnetic polarity zonation that might help determine more accurately the age of these beds. The information supplements age determination from fossil vertebrate and diatom assemblages found at this location (Scott and Weasma, 1990; Repenning, written commun., 1990; J. Platt Bradbury, oral communs. to Repenning, 1990, 1992). The Froman Ferry section of the Glenns Ferry Formation is on the south facing slope of the Chalk Hills, 23 km (14 mi) west of Nampa, Idaho, in T. 3N. , R. 4W., Sec. 8, 9, and 17 of the Marsing, Idaho, 7.5-min. quadrangle (fig. 1). There, the Glenns Ferry beds are covered by Pleistocene and Holocene gravels of the Snake River as high as ~38 m (125 ft) above the level of the Snake River. About midway up the sampled section, at the level of Hoskins Road, the Glenns Ferry beds form a broad, gently sloping, sandy bench hosting grassy fields and cultivated farmland. Upslope, the bench is interrupted by two moderately steep berms formed of more erosion resistant, finer grained sediment suitable for paleomagnetic sampling. North of Deer Flat Low Line Canal, the upper 45 m (145 ft) of the section are generally well exposed on the steep south slope of the Chalk Hills. Lithology of the Glenns Ferry Formation at the Froman Ferry locality is dominated by silts, with variable admixtures of clay and fine to medium sand. Two tephra layers, one fine sand-sized and silicic, the other coarse-grained and mafic, occur near the base of the described section (fig. 2). The section generally coarsens upward. Silty, olive-gray clay typical of Glenns Ferry lacustrine deposition is subordinate. Plant fragments are common in many beds. The beds show cyclic repetition of pond and marsh, to floodplain and fluvial facies across the section. The cycles are repeated at approximately 5 to 8 m (15 to 25 ft) intervals in the lower half of the section, but broaden substantially in the upper half (reflected in our sampling intervals). This cyclicity is a general pattern with many depositional hiatuses evident from weathered, oxidized horizons at the top of some of the clay and silt layers, and one cobble and pebble conglomerate overlying a silty clay. Sedimentary structures include mediumto large-scale cut and fill channels, crossbedding in sand beds, reversed and normal grading in silts and silty clays, and soil structures associated with clays. On an outcrop scale, bedding is horizontal, however, many beds are lenticular and discontinuous over a few tens to hundreds of feet. Methods The section and sample levels were measured with a Jacob staff and level using elevations of distinctive points on Hoskins Road and Deer Flat Low Line Canal as datums. Elevations of prominent features on the topographic map served as a general check within resolution of its 20-ft contour interval. Two stratigraphic subsections separated by about 1 km (0.6 mi) comprise the upper half of the section; these were tied together by laterally persistent, erosion resistant marker beds. The upper and lower halves of the overall section were tied together by measured elevations from the datums. Sampling A total of 53 paleomagnetic samples were collected from 12 sites (FrohA FrohL, fig. 2). Sampled beds were excavated to depths of about 1 m (3 ft) to remove slope wash and slumped material, and to identify less weathered clay and silt-size material that typically give the best paleomagnetic results. Paleomagnetic sample sites were located at or near site markers for existing fossil localities (GS201 211, fig. 2; Scott and Weasma, 1990). Where sediments were sufficiently soft, 2-cm3-volume sample containers were pressed into the outcrop. In lithified beds, samples were carved in relief, then 6-cm3-volume containers were slipped over the cubes of sediment. Samples were oriented using a magnetic compass and clinometer, then detached from the outcrop. Softer sediments are common in the lower 40 m (-125 ft) of the section. Harder sediments of the upper 45 m (-145 ft) of the section dictated the exclusive use of 6 cm3 sample containers. Stratigraphic separation between sampled beds ranges from 2.1 m (7 ft) to 18.3 m (60 ft), with more thorough coverage in the lower 40 m (130 ft) of the section (largest separation in lower half is 8.2 m (-27 ft)). Sample coverage in the upper 43 m (140 ft) of the section is broad. Paleomagnetic Measurement Remanent magnetizations were measured using a cryogenic magnetometer with a background noise level less than 1 xlO~4 A/m. Natural remanent magnetizations (NRM) were weak in all samples, ranging in intensity from 2.33 xlO~3 A/m to 9.87 xlO~5 A/m. Most sample NRMs had low 10"^ or high 10"^ A/m intensities. After evaluating responses of 12 samples to tumbling alternating-field (AF) demagnetization in peak fields of 2.5 and 5.0 milliTesla (mT), samples were routinely demagnetized in peak fields of 10, 20, 30, and 40 mT. Measurement precision (reproducibility of results between repeated measurements) generally was within 10 percent relative error throughout the demagnetization steps for each sample. Magnetization of particularly weak samples approached the background noise level of the magnetometer after partial demagnetization and data of lower precision were accepted. This degree of error does not affect interpretation of polarity. All data are presented in table 1. Paleomagnetic Results The data show three types of sample behavior through the NRM and AF demagnetization steps. The most common NRM directions were generally north declination and steep to shallow positive inclination. After partial AF demagnetization, these directions shifted to generally southern declination and steep to shallow negative inclination. Intensity of magnetization increased in some samples during demagnetization. This behavior, illustrated in fig. 3A, is interpreted to reflect a high coercivity (i.e., stable) reversed polarity magnetization with a strong normal polarity overprint incompletely removed by AF demagnetization in peak fields up to 40 mT (table 1, sites FrohB through Frohl, and FrohK). The second type of sample behavior (fig. 3B) starts with an NRM direction of southern declination and negative inclination. After AF demagnetization, remanent directions of these samples show further alignment toward southern declination and steepen in negative inclination. This behavior is interpreted to reflect a stable reversed polarity remanent magnetization with a relatively weak normal polarity overprint that is effectively removed by AF demagnetization (table 1, sites FrohJ and FrohL). The third type of behavior (fig. 3C), seen from only the lowest site sampled, FrohA, is more difficult to interpret. Initially and in the low demagnetizing fields, these samples generally show a normal polarity magnetization (northern declination and steep positive inclination) that is diminished to <10 percent to ~20 percent of NRM intensity by peak demagnetizing fields of 30 mT. Over half of the sample's NRM intensity is removed in demagnetizing fields less than 10 mT. This low coercivity indicates that this is not a stable magnetization. Remanent intensities approach the background noise level of the magnetometer after 15 or 20 mT. Further demagnetization produces no increase in intensity seen in some samples where it is explained as removal of a normal overprint that is canceling in part the primary reversed vector (e.g., fig. 3A). Vector changes during demagnetization (except for sample 9) diverge significantly from univectorial, linear decay toward the origin of the demagnetization diagrams, indicating that there is probably a weak, but higher coercivity (i.e., more stable) component of reversed polarity (fig. 3C and table 1, site FrohA). Declinations in 3 of the 5 samples from site FrohA enter the southern quadrants in the higher demagnetizing fields, suggesting reversed polarity, but the magnetization is extremely weak and uncertainty in dire

u.s. geological survey open file report/open-file report

Paleomagnetic polarity of some vertebrate fossil localities of the Glenns Ferry Formation in the Chalk Hills, near Froman Ferry, western Snake River plain, Southwest Idaho