Weird Gamma Delta Correlation

I was training alpha at O1 & O2 this morning and the session graph showed an interesting correlation between delta and gamma.  It was somewhat rare these days for me to be doing biofeedback at all, as I usually reserve it for special occasions, preferring to simply meditate instead.  Although I should mention I've put in well over 500 hours of alpha training in the past.

I had seen this pattern before, a kind of alternating between two amplitude levels, relatively often actually, in gamma, but this was the first time I had seen it in delta, and it was highly correlated with the gamma as well.  The bursts are approximately 30 seconds in length.  It would at first glance appear to be some kind of artifact, but I assure you the other bands (theta, alpha, and beta) were completely normal .

If anyone has thoughts on this, I'd appreciate it.  Delta (0.5 - 4.0 Hz) in purple, gamma (37 - 43 Hz) in orange.  These are the 30 second averages of the sums of amplitude at O1 & O2.

Subsequent sessions at P3-P4 and C3-C4 show that the pattern is primarily at the occipitals. It exists elsewhere, but is so weak I never would have noticed it without seeing the dramatic correlation at O1-O2.