What would the geologic record look like on a planet in the galactic halo?
Imagine an Earth-like planet orbiting a Sun-like star in the inner halo of the Milky Way. As a halo star, it will likely be somewhat metal-poor, having formed early in the life of the galaxy, but other than that, we can assume the planet and star to be like our own. The system lies about 4 kpc from the Galactic Center; as such, it completes one orbit of the galaxy in somewhere between 100 million and 200 million years.
While in the halo, the star is unlikely to be near other stars, but during each orbit, it passes through the disk of the galaxy twice. If the star is traveling at $\sim$300 km/s, then it should take it about 3.3 million years to travel through the disk, where it will pass by numerous other stars and other objects. I'm trying to determine if these passages will show up in the planet's geologic record many millions of years in the future. Ideally, alien geologists (with the same tools as human geologists today) would be able to use recurring signs of interactions in the disk to figure out the period of the star's galactic orbit.
I only have one vague idea: the planet would be more likely to be near a supernova while in the disk, which would cause changes in isotopic abundances in certain rock layers. However, I have no idea whether or not this is plausible, and if it would be detectable.
Would the trips through the galactic disk be apparent in the planet's geologic record? If so, how would they show up?
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