Gabbro Near Rocky Prairie Mima Mounds

On a recent trip to Pierce County and Thurston County I plotted my route between project sites to pass by Rocky Prairie on the Old Pacific Highway to check out some more Mima Mounds. I found a spot to pull over, but before checking the Mima Mounds in the prairie I noted a small hill rising up in the forest right by where I pulled out. My initial thought was that it might be a remnant esker or terminal moraine as I was within the far southern reaches of the Puget ice lobe. I meandered towards this hill and discovered it was bedrock. This was not a complete surprise as I knew there were outcrops on the upland areas to the west and southeast.

Bedrock quarry near Rocky Prairie

I was struck by how dark the rock was - nearly black. But it was wet so maybe it was just the wetness. I guessed it would either be 1) andesite as a ridge to the southeast I had been to before was predominantly andesite or 2) basalt possibly Crescent basalt or maybe even a bit of Columbia River Basalt Group (these two formations cover lots of western Washington. I was surprised when I picked up a chunk. It was very dense and heavy. A gabbro - extra iron and magnesium. Hmmm!?

I hadn't checked out the geology map of the area before this trip as my stop was simply a minor side trip between projects and I had not been anticipating looking at anything but the Mima mounds in the prairie.

Turns out this outcrop is a bit of a mystery. The Geologic Map of the East Olympia 7.5-minute Quadrangle, Thurston County, Washington (Walsh and Logan, 2005) notes this outcrop as "of unknown affinity". They further note that it is a bit heavy on the mafic elements (iron, magnesium etc.) relative to similar appearing gabbro intrusions within the Crescent, and the date they obtained is a bit on the young side relative to Crescent Formation basalts. They tentatively have correlated this unit with Grays River volcanics based on similar augite crystals (see Dave Tucker's  doty-hills-augite). But they also suggest this particular outcrop may be associated with an entirely different pile of volcanics. Apparently these younger than Crescent Formation intrusions represent a not yet fully understood period of transition from oceanic spreading or backarc volcanism to subduction arc volcanics. 

So I went from the puzzling Mima Mounds to puzzling gabbro.