The post-rock family tree inevitably leads to the Chicago Underground collective. At the heart of that group are Rob Mazurek and Chad Taylor, who, despite their endless “rock” credits, are secretly an avant-garde free-jazz duo in the Ornette Coleman model. With Boca Negra, they pilot bare-bones drums, brass, and marimbas straight into the unorthodox tunings and slippery structures of Coleman’s harmolodics, even covering his “Broken Shadows” to make the intent clear. Boca Negra can be discordant and difficult, with shifting time signatures and sparse instrumentation delaying nearly all gratification, but when “Confliction” locks together with a vicious swing to the rhythm, their unorthodoxy makes sense.