Bricks, Masonry, Asphalt and Concrete - Contaminated Ready-Mix

We recently had a 24 x 16 patio poured with a broom finish. Brownish wood particles rose to the top leaving behind small holes randomly scattered throughout the slab. We would like to know if anyone is familiar with something like this and if it is most likely from the ready-mix supplier or somehow brought in by the contractor. We also discovered more wood particles were embeded beneath the top surface after grinding it to level out some low spots. The fact that it is small particles randomly throughout the concrete seems to indicate that it must be from the supplier. I have received opinions for both scenarios, however, and would like to get a few more.

Thank you.