A new study on the conspicuous reddish-brown dolomite cliffs within the uppermost Bright Angel Formation of the Cambrian Tonto Group in western Grand Canyon, Arizona, reveal thrombolitic bioherm patch reefs; the first such reefs reported in the Cambrian Grand Canyon. These dolostones, referred to as the ‘Rusty-Brown Dolostones’ (RBDs), are exposed intermittently for ~200kms along the Grand Canyon corridor and are hypothesized to be diagenetic tongues of the overlying Muav Limestone, but have yet to be physically linked to Muav proper nor correlated via facies or paleoenvironmental description. Bounded above and below by variably interbedded glauconitic shales and siltstones, the paleoenvironmental position that these facies occupy, and the sequence stratigraphic implications therein, are unknown and difficult to discern owing to the extensive diagenetic alteration they have undergone. Here we provide new observations of Bright Angel Formation RBD outcrops in western Grand Canyon. Field observations indicate the two previously identified meter scale RBD members (Lower Tincanebits and upper Meriwitica) are separated by ~12 m of hummocky cross-stratified and convoluted glauconitic quartz arenite siltstones to sandstones variably interbedded with shales. The lower RBD contains oolitic and oncolitic grainstones to hybrid carbonate boundstones, and fines upward to dolomicrite. The upper RBD can be seen locally forming two distinct morpho-types: 1) interbedded clastic dolomite and dolomite cemented arenite sandstones that are gradational with the underlying sediment package, fine upwards to dolomicrite, and onlap onto 2) massive and cryptic dolomite mounds with distinct clotted textures, that sharply contact the underlying regressive package and protrude 1m+ above the bedded morpho-type. We interpret these mounds as bioherms, likely thrombolitic or dendrolitic, formed in a warm shallow upper shelf environment as microbialite patch reefs that were deposited during maximum transgression. The grainstones indicate shoals while the upper decreasingly clastic dolomicrite indicates calm water deposition. The intervening HCS sandstones coarsen upwards and indicate deposition during episodic regression. Microbial patch reefs in the RBDs help define short-term transgressive-regressive episodes, suggesting a more dynamic Sauk II transgressive sequence