DEPOSITIONAL HISTORY OF THE ALBIAN TO CENOMANIAN LOWER IRON SPRINGS FORMATION: IMPLICATIONS FOR EPISODIC ADVANCEMENT OF THE SEVIER FOLD-THRUST BELT IN SOUTHWEST, UTAH

The Albian-Cenomanian Iron Springs Formation (ISF) in the Three Peaks region of southwest Utah records an abrupt episode of deformation associated with advancement of the Sevier fold-thrust belt synchronous with the ~100 Ma magmatic flare-up in the Cordilleran arc. Here, the ISF was deposited along the western scarp of the east-vergent Iron Springs thrust in a basin bounded by the Iron Springs thrust to the east, and the Blue Mountain and Wah Wah thrusts to the west. The basal ISF consists of the Marshall Creek breccia, a 20m-50m thick, mostly matrix-supported, limestone breccia, the Three Peaks Tuff, a 5-15m thick airfall tuff, and the overlying “lower Iron Springs”, consisting of interbedded sandstone, limestone, and pebble to cobble, clast-supported, conglomerate. Our results include seven detailed stratigraphic sections measured through the Marshall Creek breccia, two measured sections through the lower ISF, detrital zircon analyses, and our new geologic map of the area. Six distinct lithofacies in the Marshall Creek breccia record deposition in three environments: debris-flow dominated alluvial fan, fan-adjacent lake, and braided stream. Deposition of the Marshall Creek breccia reveals the earliest movement on the incipient Iron Springs thrust. The age of the Marshall Creek breccia is constrained by its youngest detrital zircon (100 +/-3 Ma) and the age of the overlying Three Peaks Tuff (100.18 +/- 0.04 Ma). The lower Iron Springs Formation was deposited in lacustrine and adjacent fluvial environments during a period of relative tectonic quiescence on the Iron Springs thrust. The age of the lower ISF is bracketed between 100.18 Ma (age of the underlying Three Peaks Tuff) and 98.63 +/- 0.05 Ma (age of a thin airfall tuff ~44 meters above the top of the lower Iron Springs Formation).

The basal ISF record is therefore inextricably linked to local fault movement. Earliest deformation along the Iron Springs thrust was a discrete episode, occurring no earlier than 103 Ma and no later than 100 Ma, marked by deposition of the Marshall Creek breccia, followed by an episode of quiescence, represented by the deposition of the Lower ISF between ~100 Ma and ~99 Ma. Deformation along the Iron Springs thrust at ~100 Ma was generally synchronous with a thrust-belt wide thrust propagation event, and a magmatic flare-up in the Cordilleran arc.