The Paradox Basin cyclothems previously have been interpreted as Milankovitch style glacial–interglacial cycles from the Late Paleozoic Ice Age, but an unambiguous test for a glacioeustatic origin has not been conducted. A high resolution coupled chemical and physical stratigraphic analysis of two outcrop sections and three core segments provides new evidence that supports either minor sea level change of several meters or an autocyclic mechanism for parasequence formation. High amplitude sea level change is ruled out by the scale of thin top-negative isotopic meteoric diagenesis trends associated with parasequence tops and subaerial exposure fabrics. Isotopic gradients from shelf (light) to basin (heavy) indicate that parasequences are deposited diachronously, with isotopes of more distal sections recording increased basin restriction. These results support the idea that the late Pennsylvanian was a prolonged period of relatively static eustasy, agreeing with recent studies in the western USA. The methods provide a new set of tools and context for extracting environmental information from cyclic upward shallowing carbonate parasequences.