Verbal | Is this sentence parallel?

Hi guys,

As I was studying for the GMAT a doubt on parallelism downed on me. I'll exemplify it through an example that should make it clearer in case anyone more expert than me would like to jumpin:

- John contributed to archeology through new discoveries and by documenting oldones.

"Through" and "by" are both prepositions, but in this case the first one is followed by a noun while the second one is followed by a verb in the 'ing' form.
Consequently I'm not sure whether
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