Day #3 Outcrop: Mauna Ulu Flow Field

Six years ago I was on the Big Island of Hawaii leading a group of undergrads from Lake Superior State University on a geology field trip of the island. It rained on us a lot that week – no fun since we had planned to camp in tents to save money. Not that it put a damper on our desire to see geology in its natural habitat…

For today’s outcrop I’ve selected a couple of exposures of lava flows from the 1969-1974 Mauna Ulu eruptive sequence. First some nice surface forms of a’a and pahoehoe:

Nichole Peers into a Shallow Lava Tube

Pahoehoe Surrounds an Upright Block of A’a.

Along the shoulder of Chain of Craters Road, it’s now possible to see a cross section through the upper part of this flow field, including some lovely lava tubes.

Lava Tube, Geologist for Scale

Successive Coatings of Basalt Fill this Tube

And finally some flora, for the biologists out there. ;-)

Ohia Lehua

Life Will Find a Way

Day #2 Outcrop: Navajo Sandstone, near Rifle Falls, Colorado

This is a series of field shots of a lovely little roadcut through Navajo Sandstone on CO 325 about two kilometers south of Rifle Falls State Park shot during FHSU’s Summer Field Camp in May, 2007.

Navajo Sandstone Outcrop in the Ridge Above the Roadcut

And now a little closer, so you can see what caught my eye as we originally drove past.

IMG_5452 IMG_5458
Look at those Beautiful Aeolian Crossbeds!

Sure, I’m a hard rock geologist, but who wouldn’t be moved by sedimentary structures as beautiful as these.

Textbook Crossbeds
Too Close!

More where these came from.

Day #1 Deskcrop – Garnet-bearing Trondhjemite

I’m starting off the year with one of my more uncommon deskcrops. This is a piece of garnet-bearing trondhjemite collected from the German Rancho Formation, near Stillwater Point State Park, in the Gualala block of coastal northern California. This rock was one of the most distinctive, and yet confounding from my Ph.D. thesis area. The view here is the cut face of a cobble-sized conglomerate clast that was deposited during the Eocene in a submarine fan channel deposit somewhere west of North America. The exact provenance of this particular type of clast could not be established because no exact matches are currently exposed in any of the likely source areas.

Garnet-bearing Trondhjemite Hand Sample

When I first encountered this type of clast in my first field season back in the summer of 1992, I immediately recognized little (1-3 mm) red soccer ball garnets and presumed the rock was some sort of metamorphic rock. Upon returning to Wisconsin and thin sectioning the rock the following fall I was surprised to discover that its mineralogy and petrography were distinctively igneous. Chemical analysis confirmed that this rock was a Fe, Mg, K-poor, Na-rich granitoid most resembling the plutonic rocks known as trondhjemites. As it turns out, the garnet was not Mn-rich as might be expected in such a leucocratic rock.

garnet trondhjemite thin section - PPLgarnet trondhjemite thin section - XPL

Such a distinctive rock seemed to be exactly the sort of clast one would hope to identify in a provenance study – one that might possibly be used to identify a unique source pluton and establish a sort of sedimentary “piercing point” to constrain early offset on the San Andreas and related faults. Only we never found the source pluton. Based on geochemical signature and U/Pb crystallization age of this clast and others like it (Early Cretaceous), and the distribution of geochemically similar rocks in the Cordilleran batholiths of western North America, and in consideration of other clasts from the basin, it seems most likely that the parent of this orphan is currently buried beneath sediments of the Great Valley Sequence or the Pacific Ocean along the western margin of the Sierran/Salinian batholithic belt, in a resting place currently known but to God.