SDAG Monthly Meeting - Virtual
Wednesday - January 19, 2022
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Location: another virtual meeting in front of your computer or phone
SDAG Monthly (Virtual) Meeting
6:30pm - Happy Hour
7:00pm - Meeting begins
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Meeting ID: 857 9818 4239
Passcode: 461844
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6:00pm -
Social hour |
Menu: What's in your fridge?
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Cost: it's virtually FREE
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8:00pm -
Program
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"Extending the Busch-Miller (2016) Hypothesis: Acid Rain from K-Pg Bolide Impact Chemically Altered Exposed Igneous Rocks in San Diego County"
Speaker: Eleanora (Norrie) Robbins, PhD
USGS - Retired / SDSU - Retired
Busch and Miller (2016) recognized that Paleocene sediments in California are characterized by the presence of kaolinite. They hypothesized that the K-Pg bolide that impacted anhydrite beds in Yucatan created worldwide acid rain, resulting in intense weathering of exposed rocks and sediments, thus forming kaolinite. They assessed kaolinite in the Simi Conglomerate, Silverado and Goler Formations, and the basal units of the Ione, Walker, and Maniobra Formations. Their hypothesis suggests that kaolinite may provide a good search image for the boundary.
The K-Pg event has been identified in 107 sites (so far) around the world. The sites in San Diego County where the K-Pg might be encountered are the Lusardi Fm in places like Poway, Alpine, Carlsbad, 4S Ranch, as well as Mission Trails Regional Park and Camp Pendleton.
Working on Camp Pendleton in 2012, Andy Pigniolo, Greg Cranham, Bill Elliott, and I published our study of the Piedra de Lumbre chert (PDL) in the SDAG 'Waiting for Tsunami' guidebook. The chert and its enclosing 'porcellanite' have clasts of quartz that makes this unit archeologically distinct. While the PDL was a hot spring deposit, there still isn't a great explanation for the presence of the distinctive scattered quartz clasts. This unit, indurated by silica, sits on the Late Cretaceous Williams Fm. Neither chemistry nor clay mineralogy of the porcellanite have been analyzed, but kaolinite is a possibility.
The Lusardi Fm is turning out to present another silica mystery. The Lusardi has no fossils and sits directly on granitic rocks of the Late Cretaceous Peninsular Range Batholith (PRB). In Alpine and Poway, the outcrops are so sinuous that it has been easy to explain them as fluvial, even though there are giant boulders in the otherwise arkosic sandstone; Peterson (1971) defined these Lusardi beds as having been created in very fast flowing turbulent rivers. Torrential rainfall and mudflows could account for these. The rainfall has been estimated to have lasted between a few months to a few years. That sounds like enough water to have moved boulders creating the Lusardi Fm, which is then potentially a deposit of the K-Pg impact events.
Our current study is one of reconnaissance of the Lusardi Fm. The boulders are PRB granitics and Santiago Peak Volcanics (SPV) andesite. Boulders, cobbles, and pebbles have distinct white weathering rims, some of which resemble the porcellanite at Camp Pendleton. Neither chemistry nor clay mineralogy have been assessed yet, but kaolinite is a possibility.
The Lusardi Fm contains clasts that are easy to mistake for flint. Indeed, 'pawii' is the Kumeyaay word for arrowhead, flint, and chert, and the source of the name of the City of Poway. These clasts are siliceous, flow-banded, and black. The rock type is so distinct in archaeological lithic assemblages that it has its own designation, 'Lusardi Fm Volcanic.' Outcrops of the obvious source of this finely crenulated flow-banded SPV welded tuff were pointed out by Monte Marshall on the 2020 SDAG field trip up Iron Mt., about 6 km due east of our Poway study site.
The idea that the K-Pg was a time of intense acid rain that kaolinitized the rocks sitting at the surface at that time is so intriguing that our study is focused on being a test of this hypothesis. Someone with XRD and clay mineralogy expertise or desire to become an expert is needed to advance the research.
Norrie is an economic geologist, paleo-palynologist, and geomicrobiologist. Her degrees are from Ohio State, Univ. Arizona, and Penn State.
Her work life included 2 years as a Peace Corps Volunteer with the Geological Survey of Tanzania, 34 years with the USGS in Washington D.C., Denver, and Reston, as well as 15 years as adjunct faculty at SDSU in the Dept. of Geological Sciences.
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Upcoming SDAG meetings - 2022
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