The first precise, biostratigraphically bracketed U-Pb dates on the middle Middle Cambrian come from the Rte. 111 ash (new) in the lower Manuels River Formation of southern New Brunswick. Manuels River black mudstone (Avalonian depositional sequence [Ads] 8) unconformably overlies Fossil Brook Member greenish mudstone (Ads 7) of the Chamberlain’s Brook Formation, and the two units should not be combined into a ‘Forest Hills Formation’ (abandoned). This unconformity marks the trans-Avalonian (i.e., Rhode Island–Belgium) green–black boundary and onset of ca. 26 Ma of dysoxic/anoxic marine deposition. Trilobites and agnostids correlate the surprisingly endemic, upper Paradoxides abenacus Zone and Rte. 111 ash into the Hypagnostus parvifrons Zone (Drumian) in Avalonian Wales, Baltic upper ‘Acidusus’ atavus Zone and upper Mawddachites hicksii–lower Paradoxides davidis zones of Avalonian Newfoundland and Britain. The Manuels River Formation in SE Newfoundland and coeval Nant-y-big Formation in South Wales have not yielded ash dates. However, our U-Pb zircon analyses of the Rte. 111 ash in the lower Manuels River in southern New Brunswick yield statistically identical ages of 501.44 ± 0.10 and 501.45 ± 0.08 Ma. The Ads 7–8 unconformity is locally 6–10 m lower in New Brunswick and is somewhat older. Our ages for the Rte. 111 ash show the lower Drumian is significantly younger than previous estimates, is ca. 4.9 Ma younger than the Lower–Middle Cambrian boundary, and debunks claims of a continuous Lower–Middle Cambrian succession in Avalonian New Brunswick where a ca. 7 Ma hiatus is present.