And more particularly quite where.
We think we’ve struck gold when we find “Field geology of NSW” by Branagan and Packham, and use it to shape a Sunday explore in search of aspects of Narooma Terrane (aka Narooma Accretionary Complex) that we haven’t been aware of before. Specifically, we’re hunting for BIM mélange. Wikipedia provides us with a photo (no scale) and some basic information. I’m most struck by the fact that this mélange probably rode the Pacific tectonic plate at least part of its 2500 km journey towards the east coast of Gondwana some considerable time ago.
At this point we have a vague unformed idea of what we’re looking for. The next thing we need to know is where to find it. And that’s when local knowledge and the handbook come to the parting of the ways. We know exactly where pillow lava is, one of our few certainties in this geology game. But the headland nomenclature, always a bit shaky, doesn’t match. We follow written directions like good little students and although we find fascinating rockfaces that raise another lot of questions (Does that look like chert? Do you reckon this could be basalt? Is this side of this headland the same as the other side, only differently weathered?) we don’t find any sign of the imbricate stack. So we go a couple of beaches south to the patch of known pillow lava, and poke around muttering imprecations and mumbling interrogatively. We scurry around a headland between waves, and find what we’re looking for in Smugglers Cove.
Mind you, we were looking for a neat imbricate stack (don’t ask!), but we find BIM mélange that perfectly matches the Wikipedia photo (the first in the pairing below), the bottom layer of the imbricate stack sequence – if we’ve got it right. We give ourselves 50% for the morning’s work, and head home sun-soaked and weary.
What exactly is block in matrix mélange? As I search for an easy definition I’m back in the realm of every second word a mystery that needs to be solved. Breccia, tectonic accretionary prisms, olistostromal action, orogeny, boudinage, dilational veins, mylonite, imbricate stack. Talk about an accretionary zone in vocabularics! It’s a relief to meet a few old familiars, even if I’m not completely certain of their meaning: Lachlan Fold Belt, subduction, turbidites, chert. They’re like old friends spotted at a party filled with strangers.
Oh, and BIM mélange? A sedimentary deposit composed of a chaotic mass of mixed material turned to stone. I think.