Abstract

Continental strata and volcanic rocks of the type Huizachal Group in Valle de Huizachal record arc magmatism and subsequent crustal extension prior to seafloor spreading in the Gulf of Mexico. The older La Boca Formation consists of two informal members, a lower unit of siliciclastic and volcanic rocks discordantly overlain by a predominantly siliciclastic upper member. The younger La Joya Formation is an upward-fining, alluvial-braided fluvial succession with a basal conglomerate.

U-Pb detrital-zircon ages (n = 576) from six Huizachal Group sandstones (five from La Boca and one from La Joya) consist of four groups indicating a mixed provenance: (1) Grenville grains (∼1.3–1.0 Ga) derived from Gondwana (Novillo Gneiss); (2) early-middle Paleozoic grains (430–300 Ma) derived from peri-Gondwanan accreted rocks (Granjeno Schist); (3) Permo-Triassic grains (296–222 Ma) derived from volcanic and plutonic rocks (West Pangaean arc) and/or turbidites (Guacamaya Formation); and (4) Early-Middle Jurassic grains (199–164 Ma), locally derived from the Nazas arc. Groups 1–3 increase in abundance upsection as a result of unroofing of Jurassic volcanic and sedimentary carapace from uplifted basement.

The Huizachal Group records three stages in the pre-breakup history of Gondwana: (1) The lower member of La Boca Formation (maximum depositional age 184–183 Ma; Pleinsbachian) indicates Nazas arc activity; (2) the upper member (maximum depositional age 167–163 Ma; Bathonian–Callovian) indicates continued arc magmatism as early crustal extension formed horsts that supplied basement grains to an incipient rift basin; and (3) the La Joya Formation represents late rift basin development and widespread exposure of flanking basement rocks. Although our La Joya sample lacks a coherent age group of young grains, its single youngest grain age (164 ± 3 Ma; Callovian) is consistent with its stratigraphic position beneath inferred Oxfordian strata.

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