Longtime followers of avant-garde jazz may be aware of the importance of the Chicago-based Delmark label and its storied representation of many of the giants of Windy City adventurism such as the Art Ensemble of Chicago and Anthony Braxton, as well as its advocacy of the blues scene. But unless they've been paying attention, they might have missed the rebirth of the imprint as an avid reporter of the latest generation of jazz heroes in Chicago, many of whom are 20 to 30 years younger than a Roscoe Mitchell or a Fred Anderson.

Lately, Delmark has documented the likes of leaders such as guitarist Jeff Parker (of Tortoise), trumpeter Rob Mazurek (of Chicago Underground configurations), and pianist Jim Baker, among others.

They've also stepped up to the plate for tenor saxophonist Keefe Jackson, whose ensemble Chicago-Luzern Exchange is named after a "sister cities" program in which Swiss tuba player Eric Unternahrer traveled to the States and interacted with Chicago musicians. Unternahrer's new musical compadres also included cornetist Josh Berman and drummer Frank Rosaly (who migrated from Arizona in 2001).

Together, the Exchange, which performs Wednesday at Modern Formations Gallery, became a formidable jazz quartet, but not the traditional kind you might hear on WDUQ. Instead, its interests tend toward the realm of free and whimsical improvisation, spinning out mostly short vignettes (there are 19 tracks on the Exchange's 2005 album "Several Lights") that explore the myriad permutations of both individual instrumental tonality and spontaneous interaction.

That is to say you won't find any swing, easily recognizable heads, or straight-ahead rhythmic propulsion on "Several Lights." For example, on "Slips," the players travel over, above, and around each other (a la the Rova Sax Quartet's finest moments), while Rosaly offers a skittery percussion line that moves in and out. On "Five Handfuls," Jackson steps almost immediately into a free-squonk zone with his bandmates, equal to anything from the classic '60s era of free jazz, when it was all so wondrously new and liberating.

Unternahrer provides a steady low end and is surprisingly contemplative on a track such as "Walls," but there are times when he deliberately resembles a rumbling elephant (such as on "Trouble") more than a tuba, while Berman invites favorable comparisons to Don Cherry for his spry, sophisticated brightness. But don't get the impression that these guys run out of creative steam after a couple minutes -- the centerpiece here is "Take The Place," a tempered, 12-minute voyage into breathy sounds, pointillistic use of space, mindful dissonance, and a nearly telepathic repartee of ideas that never disappoint for the entire duration even while building to a crescendo and back again to "lowercase" subtlety.

That piece, as well as some others on "Several Lights," come across like carefully planned-out, complex chamber compositions even though they are totally improvised. Which is a testament to how well these guys work together -- it's an exchange that has clearly borne fruit.

The concert is at 8 p.m. Wednesday at Modern Formations Gallery, 4919 Penn Ave in Garfield. Openers are Rick Gribenas, Ben Harris, and Bernabo/Beyer/Masterman; $7, 412-362-0274.

-- Manny Theiner for the Post-Gazette