HIGH-FLUX VOLCANIC EVENTS OF SOUTHERN CORDILLERAN MAGMATIC ARC RECORDED IN FOREBULGE AND BACK-BULGE DEPOSITS OF THE NORTH MEXICAN FORELAND BASIN (LATE CRETACEOUS), NORTHESTERN MEXICO

Two major magmatic events that occurred in western Mexico during Late Cretaceous time are recorded in the North Mexican Foreland Basin (NMFB). We compared the distribution and ages of plutonic rocks from the southern Cordilleran magmatic arc with zircon U-Pb geochronological and geochemical data from contemporary neighboring foreland basin strata to evaluate the effects of the magmatic arc on the sedimentary fill of the NMFB. Detrital Zircon (DZ) U-Pb ages (n = 889) from ash beds and volcaniclastic sandstones were obtained and compiled from sedimentary successions in the NMFB. The proximal foreland succession consists of calciturbidites that interfinger westward with siliciclastic turbidites in the Mesa Central. New DZ U-Pb ages from two sandstone samples suggest initial basin deposition in the Cenomanian with a principal age peak of ~96 Ma. More than 12% first-cycle DZ ages of ~96-92 Ma (early Cenomanian-Turonian) indicate that much of the sediment was derived from contemporaneous magmatism during this initial, short-lived, but voluminous magmatic event. Distal foreland strata consist of carbonate pelagites with abundant tuffs and volcaniclastic sandstones of the Caracol and San Felipe Formations. Thick fine-grained facies are indicative of deposition in the forebulge and back-bulge, respectively. These units form an arcuate outcrop belt, ~700 km in length, that runs parallel to the present deformational front of the Sierra Madre Oriental of eastern Mexico. These U-Pb zircon ages between ~86 and 74 Ma (Santonian-Campanian) record a second prominent magmatic event and sandstone composition points to a dominant western Mexico arc source.

Granites in Sinaloa, Baja California and Jalisco with ages that match the age ranges of the foreland strata are considered to be the plutonic counterparts of the volcaniclastic detritus. Most sediment was transported from multiple, simultaneously active sources. Volcanic material likely arrived in the distal foreland as ash clouds from explosive eruptions, transported by prevailing Late Cretaceous northwesterly winds. We infer that magmatic flare-up events were of large magnitude (areal dimension of 100s-1000s km²) and duration (~10-15 Ma) and produced the volcanogenic strata of the Caracol and San Felipe formations in the NMFB.