MULTIPLE BASALT SILL INTRUSIONS INTO THE MIOCENE PAYETTE/DRIP SPRINGS AND LOWER CHALK HILLS FORMATION SEDIMENTS, 1.4-2.4 KM DEEP BENEATH ONTARIO, OREGON: IDENTIFICATION AND SIGNIFICANCE FOR WESTERN SNAKE RIVER PLAIN STRATIGRAPHY

The 3.065 km deep Ore-Ida geothermal well contains 6 basalt layers in the interval 1.39-2.46 km interpreted as basalt sills intruded into the Payette/Drip Springs and lower Chalk Hills Fm sediments. Logs show increased resistivity of mudstone above and below the basalt layers, giving the resistivity logs a flattened dumbbell shape (profile of the London Tower Bridge) centered on resistive basalt. All the basalt sills have similar natural gamma log count of ~65 API, and vary from 4 to 80 m thick. Petroleum well logs show the sill field extends into Idaho and covers an area of at least 230 km². Analysis of 9 selected basalt samples of Ore-Ida cuttings from 1.4 to 2.4 km deep are all quite similar and plot in the field of Snake River olivine tholeites (SROT) with high TiO₂ (3.33-3.94%) and FeO(15-15.7%), MgO (4.7-6.4%), SiO₂ (44.7-47%). SROT is considered to have erupted more recently than 11 Ma. 2.46-2.82 km is massive basalt with low gamma log count of 20-30 API, below which are interlayered basalt (gamma of 50 API) and rhyolite (gamma of 220 API) to total depth of. Chemistry of units below 2.46 km is in progress. Sediment above 762 m depth is lacustrine mudstone and minor sand of the Glenns Ferry Formation. From depth 762 to 1.43 is the thick mudstone unit of the upper Chalk Hills Formation that is the confining layer for oil and gas production in adjacent Idaho. Below 1.43 km is a 178 m thick siltstone unit with minor sands, intruded by the uppermost 24 m thick sill and interpreted as the lower Chalk Hills Formation. All sediment above 1.61 km depth shows SW dip of 4-20° on the dipmeter log. Sediment from 1.61-2.38 km shows different dips of SE, E, and NE 8-40° and is interpreted as Payette/Drip Springs Formation, 520 m thick unit when sill thicknesses are discounted. The unit is a 320 m thick lower sequence fining upward from tuffaceous claystone to fine sandstone; overlain by 3 sequences, ~ 60 m thick each fining upward from tuffaceous claystone to medium and coarse sand. Deposition is believed to precede formation of the western plain, and may have been in N-S trending basins associated with the Oregon-Idaho graben.