18 May 2012

Friday fold: Bedford Canyon Fm., California

Posted by Callan Bentley

Another guest submission for the Friday fold:

Callan –

Please find enclosed a collection of photos you could possibly use for a future Friday Fold blog post. The following photos were all taken by me on Sunday, April 29th, in the vicinity of Bedford Peak and the Maple Springs Trailhead (Silverado Canyon Road), Cleveland National Forest, Santa Ana Mountains, Orange County, California.

The rocks shown are sandstones/meta-sandstones and shale/slate of the Jurassic Bedford Canyon Formation, deposited as deepwater turbidite beds.

Most of these photos were taken along the Bedford Peak Trail.

Mike Huggins, Irvine, California

There is a USGS Open File map (Corona South Quadrangle) that covers part of the area that Mike traversed when taking these photos (extreme southern part of map, mapped in gray as Bedford Canyon Fm., trail on map [in part] has a multitude of dip/strike marks around the 2600 ft contour).

Description (Gray, et al): Bedford Canyon Formation (Jurassic)—Slightly metamorphosed assemblage of interlayered argillite, slate, graywacke, impure quartzite, and small masses of limestone. As used here, Bedford Canyon Formation is areally limited to northern part of Santa Ana Mountains. Most of unit is poorly exposed; best exposures restricted to road cuts. Bedding and primary sedimentary structures are commonly preserved, although commonly distorted by tight folding. Very fine-grained fissile, black argillite and slate form beds 2 to 8 cm in thickness.  Massively bedded, impure, fine-to medium-grained, pale gray to pale brown quartzite and graywacke beds are 4 to 30 cm thick; locally carbonaceous. Lenses of conglomerate occur sparsely through sequence. Incipiently developed transposed layering, S0, is locally well-developed.

Got it? Good. Now for the pictures!

Outcrop exposure of Bedford Canyon Fm. rocks along Silverado Canyon Road about a quarter mile from the Maple Springs Trailhead:

From the beginning of the Bedford Peak Trail (not far from the Maple Springs Trail) with outcrops of the Bedford Canyon Formation on the left:

View of Bedford Peak trail from which these photographs were taken:

Close-up of alternating Bedford Canyon Fm. turbidite layers. This may be stratigraphic up, if wavy bed bottoms do mark the base of the beds; supposedly, many of the exposed Bedford Canyon rock layers are overturned. The “sandstone” rocks that I looked at appeared to be “dirty” quartzites with welded grains; they are heavily fractured and jointed perpendicular to bedding (as seen here). The intervening “shales” – probably really slates – are greatly fractured:

Faulted Bedford Canyon rocks:

And now for the folded Bedford Canyon rocks along the trail:

There’s also a short, shaky video on Flickr by someone else (“notfrisco”) here. It shows one of these outcrops.

Thanks, Mike, for your contributions. Lovely rocks indeed!