Isn't that newer excavation for the mat foundation under the core, as Plokoon11 suggested? I know he thought it might not be in the right location for that, but from the photos, it appears to me that it
is on the eastern portion of the site, where the core will be. It also appears that this newer excavation is
below the parking level in the cross-section posted by shadowbat2. And, it appears to be primarily in rock (perhaps bedrock?) based on the presence of the hoe ram and the large chunks of broken rock throughout. At least that's what it looks like to me.