Location: Stanford University
Anyone wishing to attend the lecture only is welcome at no cost.
This will be the 336th meeting since 1954.
S2 occurs in several structurally repeated belts in the southern part of the BGB. It rests directly on cherts or komatiites at the top of the Onverwacht Group and is overlain by fine tuffs, iron-rich sediments, and cherts of the Fig Tree Group. A thin ash layer about 15 m higher in the section has been dated at 3,256 Ma. S2 consists of spherules mixed with locally eroded chert and volcanic debris, but still shows Ir values up to 67 ppb. Deposition of spherules was accompanied by erosion of the older sediments, in some areas forming chert-clast and chert-block breccias up to 10 meters thick, again probably reflecting impact-generated tsunamis.
S3, the most widespread unit, occurs 50-100 m above the base of the Fig Tree Group in the southern part of the BGB but marks the base of the Fig Tree Group in northern areas. It is 3,243 Ma in age. S3 widely consists of nearly pure spherules and shows a major iridium anomaly, locally reaching 145 ppb Ir. Pronounced facies changes reflect deposition under shallow- to deep-water conditions. Shallow-water sections also show evidence that a major tsunami accompanied deposition of S3.
S4, identified in only a single outcrop, is a 5-10 cm-thick layer interbedded with current-deposited conglomerate and sandstone within a fan-delta sequence. It includes up to 160 ppb Ir.
The presence of several thick spherule layers representing relatively large impactors suggests the probable presence of many more, smaller impact-generated deposits that, collectively, offer the prospect of directly documenting the impact flux on the Archean Earth. The coincidence of the two thickest impact layers, S2 and S3, with the change from anorogenic mafic volcanic to orogenic sedimentary regimes at different times in different areas suggests that impacts may have played a significant role in pre-3.0 Ga tectonic systems.
Donald Lowe is a member of the faculty in the Department of Geological & Environmental Sciences in Stanford University's School of Earth Sciences. He did his undergraduate work at Stanford 1960-64, got his Ph.D. at the University of Illinois in 1967, taught one year at U of I, did a two-year postdoc with USGS from 1968-70, taught at LSU from 1970-88, and has been back at Stanford from 1988-present. Don's interests are surface conditions on the Archean Earth and deep-water clastic depositional systems.
If you want to pay in advance:
Stanford faculty and students: Please make dinner reservations by Friday, April 6. Contact Dr. Juhn Liou via his mailbox (and leave check), Geological and Environmental Sciences Office, Geocorner - Bldg. 320 (Rm. 118). Make checks out to "PGS."
All others, including faculty and students from other Bay Area universities and colleges and USGS: Please make dinner reservations by Friday, April 6. Contact Willie Lee, at USGS, MS-977, 345 Middlefield Rd., Menlo Park, CA 94025, phone 650-329-4781. Send check made out to "PGS" to Willie.
Dinner is $26.00 (note price increase). Includes wine (5:30 to 6:15 PM.), dinner (6:15-7:30 PM.), tax, and tip. Note: PGS does not make revenue on this price.
For students from all universities and colleges, the dinner, including the social half-hour, is $5.00 and is partially subsidized thanks to the Associates of School of Earth Sciences, Stanford University (Note, no-show reservations owe the full price).
Dues for Academic Year 2000-2001 ($10.00) should be sent to Willie Lee, USGS, MS-977, 345 Middlefield Rd., Menlo Park, CA 94025. Willie's phone: 650-329-4781.
Officers: Gary Ernst, President; Mike Diggles, Vice President; Vicki Langenheim, Secretary; Willie Lee, Treasurer
Date created: 03/07/2001
Last modified: 04/10/2003
Created by: Mike Diggles, Vice President, PGS.