NEW AGE DATES FROM GRANDE RONDE LAVAS, COLUMBIA RIVER BASALTS: IMPLICATIONS FOR PERIODICITY OF FLOOD BASALT ERUPTIONS AND DURATION OF MAJOR PULSES

Grande Ronde Basalt (GRB) lavas represent the most voluminous eruptive pulse of the Columbia River-Snake River-Yellowstone hotspot volcanism. With an estimated eruptive volume of 150,000 km³, GRB lavas form at least 66% of the total volume of the Columbia River Basalt Group. New ⁴⁰Ar/³⁹Ar dates for GRB lavas reveal they were emplaced within a maximum time range of 0.40 ±0.2 My. A well-documented stratigraphy indicates at least 110 individual GRB eruption packages, or flow fields, and the age data suggest an average inter-eruption hiatus of less than 4,000 years. Successive eruptions varied considerably in volume but time-volume relationships suggest many GRB eruptions were greater than 1000 km³ in size. Radiometric age-dating cannot resolve time gaps between GRB eruptions, yet it is difficult to form a picture of the lengths of them by other methods because even the same two flow fields display variable weathering of the lower unit and/or amounts of sediment at the contact at different locations. Except for locations around the edge of the province, these contacts have no sediment to a few cm of silty to fine-sandy material (possibly eolian), with occasional diatomaceous sediment (also sometimes eolian) and cherts, and, rarely, 20-30 cm of the same silty-sandy material (possibly fluvial) and in-growth-position tree molds. Most probably eruptive events were not equally spaced in time; some eruptions may have followed short periods of volcanic repose (perhaps 10² to 10³ of yrs), whilst others were considerably longer (many 1000s to > 10⁴ of yrs).

Recent improvements in age-dating for other continental flood basalt (CFB) lava sequences have yielded estimates of total eruptive durations of less than 1 My for high-volume pulses of lava production. GRBs appear to be a similar example, where the main pulse occupied a brief period. Even allowing for moderate to long-duration pâhoehoe flow field-producing eruptions, the amount of time the system spends in active lava-producing mode is small (for the GRB from perhaps > 1000 yrs to 10,000 yrs) compared to the duration of the entire eruptive pulse (c. 400,000 yrs). Resolving time intervals between successive eruptions during CFB province construction, and durations of main eruptive pulses, remains vital to determining the environmental impact of these huge eruptions.