Establishing itself as a cable phenomenon as well as the biggest thing ever to hit Comedy Central, the off-kilter animated series “South Park” on Wednesday night obliterated the viewership record it had set on the network just three weeks before by pulling down a sparkling 4.8 rating for the show’s Thanksgiving-themed episode.The number bettered by a full point the 3.8 rating that “South Park” generated for its Halloween episode on Oct. 29 — setting the all-time record for the all-comedy service until Wednesday. Before “South Park” came on the scene, Comedy Central’s highest rating was a 2.7 for an episode of the British import “Absolutely Fabulous.” The network typically attracts roughly a 0.7 rating in primetime, so the 4.8 for “South Park” — which airs Wednesday nights at 10 — improves on the average nearly seven-fold. The 4.8 figure is likely to stand as the highest non-sports program on basic cable for the week. The Wednesday episode, which finds the “South Park” gang adopting an Ethiopian child, was seen by an estimated 3.7 million viewers in Comedy Central’s viewer universe of about 45 million homes. “South Park’s” number on Wednesday is all the more impressive considering that it came in the middle of November sweeps at the broadcast webs. ‘South Park’ sketch on Leno The show about a group of kids and their misadventures in a deceptively quiet Colorado town was skedded to get its widest exposure last night, when “The Tonight Show With Jay Leno” featured an animated Leno in a two-minute sketch with the “South Park” characters. “South Park” was honored with the CableAce Award as cable’s top animated series on Nov. 14.