Most sources cite that July 1997 marked the end of Cartoon Network's original Checkerboard Era, and the beginning of the well-known and loved Powerhouse Era. But that's not entirely accurate.
In actuality, July 1997 brought a brief mini-era of Cartoon Network history that lasted into the spring of 1998, and is all but forgotten - probably because unlike either Checkerboard or Powerhouse, this mini-era was not only rather generic and lacked any kind of personality or distinction, but it was also a transitional period as well - old Checkerboard Era bumpers, promos, and IDs were still being phased out up until March 1998, while Powerhouse Era was slowly being introduced beginning in December 1997.
Perhaps this transitional mini-era could be known as the Starburst Era, as a majority of the new promos for this era were for the then-new Cartoon Cartoons (Johnny Bravo, Cow and Chicken, and a new season of Dexter's Laboratory), and featured the Cartoon Network logo plastered over a colorful starburst. These promos also often featured an electric guitar riff that ended with a 12-note stinger accompanying the aforementioned starburst logo closing. A majority of these promos were animated by Todd Wahnish, who animated all of the Powerhouse Era bumpers and IDs.
Because this marked the beginning of Cartoon Cartoons, the new shows were promoted heavily during this time, with most of the promos lasting well into the spring of 1998. Originally, the Cartoon Cartoons would open with a green version of the intro, featuring a different variety of cartoony voice-overs announcing "CARTOON CARTOON!" These continued to be used to open Cartoon Cartoons up until November 1998, when the custom, show-specific, blue intros were introduced as part of the 1998 Cartoon Cartoon Weekend. For the December 1997 Cartoon Cartoon Weekend, the green intros would feature random Cartoon Cartoon characters popping in and out of various different blocks in the Cartoon Network logo.
As noted above, this mini-era was transitional. While the Cartoon Cartoons had their own series of bumpers and network IDs, many older properties, such as Warner Bros. cartoons (Looney Tunes and Merrie Melodies) and Hanna-Barbera shows still used their Checkerboard Era bumpers and IDs up until March 1998, when they began adopting their own Powerhouse Era bumpers and IDs. A couple of weeks after the inception of Cartoon Cartoons, most Cartoon Network shows retired their Checkerboard Era Strings "Next" bumpers in favor of "Next" bumpers that opened a series of brightly colored and funky-designed curtains - these also continued to see use until March 1998 when Powerhouse Era bumpers were being introduced (some minor exceptions, such as The What A Cartoon! Show still retained their Strings "Next" bumpers during this time).
Perhaps the earliest-known example of Powerhouse being incorporated into Cartoon Network was the Great Crate Contest that lasted from October into early November 1997 - the promos and commercials for this contest featured the familiar arrangement of Raymond Scott's "Powerhouse" song that was associated with many of the Powerhouse Era bumpers, promos, and IDs. Late December 1997 saw the introduction of the Cartoon Crisis Center spots, which could arguably be considered part of the Powerhouse Era, as these spots continued to see use throughout the era. Beginning in mid-March 1998, Todd Wahnish's Powerhouse promos, bumpers, and IDs were gradually being introduced, while any remaining Checkerboard Era spots were finally being put to rest. By April 1998, Powerhouse Era had officially taken over, and any traces of Checkerboard Era were completely gone.