Tuesday, April 1, 2008

Death Valley Day 2.0 – Furnace Creek Fm

The Miocene-Pliocene Furnace Creek Formation consists of fluvial and alluvial fan conglomerates, graded sandy turbidites (dominantly T a-b divisions) deposited in a lake, and finer grained lacustrine deposits. It is exposed in several really nice spots in Death Valley, though by far the best exposures are found at the Hole-in-the-Wall campground. The campground is out of the way; you have to drive up a dry river channel to get to it, and it abuts a wilderness area. In my opinion, it is the best campground in the park, for the simple reason that it has incredible exposures of the Furnace Creek Fm.

All the way in the back, near the wilderness area, there is an amazing exposure of conglomerates (the unique weathering pattern is what gives this campground its name of hole in the wall). We camped in Hole in the Wall, and started our second day looking at these strata.

The lowermost clast-supported cgl show evidence of imbrication, are normally graded to ungraded, and interfinger with sandstones showing trough x-stratification, some ripple formsets (on bedding surfaces), and laminated mudrock. As you move up-section, the cgl beds get thicker, and become matrix-supported. Inverse grading becomes dominant, and the clasts are more angular. They really show classic evidence for debris flow processes. To me, it seemed that these active debris flow lobes were part of an alluvial fan complex that adjoined a lake basin.

The picture below shows one of these individual debris flow beds (a single depositional event) THINNING OUT as you move from the left to the right (the kink in the bed is structural deformation related to uplift). How cool is that! You can see the evidence for the flow thinning and freezing!

The picture below is a close up of the distal toe of the above bed; note the inverse grading and presence of outsized, floating clasts at the top of the bed (younging is to the right).

Heading down the road a bit, we encountered the more distal lacustrine beds of the Furnace Creek Fm. The lake environments are dominated by sand-rich turbidites (and maybe a hyperpycnite, here or there; there was some evidence of inverse-to-normal grading within individual beds), massive mudrock (deposited from turbidite plumes?) and laminated claystone (suspension fall-out). I wonder what the relationship of these turbidites is to the debris flows on the adjacent alluvial fan? Could these be sourced by debris flows hitting the lake, entraining water, and transforming into turbulently supported flows? Alternatively, maybe they represent winnowing and reworking of the fan surface during subsequent sheet-flood events, and are only entraining the sands? I reckon detailed petrography and mapping would be the only way to start wrestling with that question.

The picture below is a set of three of these graded beds.

This next picture (below) shows some nice soft-sed deformation (ball and pillow structures, some flame structures) developed where the sand was rapidly deposited on the mud.

Finally, here is a picture of some mud rip-ups in one of these turbidites.
There were many tens of meters of beds just like this; the ol' Furnace Creek Lake must have been getting hit pretty hard with these flows. Interestingly, there is no evidence for evaporite deposition at this location (i.e., displasive crystals, evaporite mineral casts), whereas at other points in the basin, these same lakes do show evaporative evidence.

Well, that does it for the morning of the second day; later, I’ll post some pictures of tufa pinnacles.


Silver Fox said...

Interesting deposits and sed structures.

BrianR said...

Nice summary ... beautiful rocks!

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