Improved chronologic resolution of the Hawthorn and the Alum Bluff Groups in northern Florida: Implications for Miocene chronostratigraphy

Abstract

Several geochronologic methods are used to constrain the ages of the early to middle Miocene Hawthorn and Alum Bluff Groups in the eastern Florida panhandle. The Dogtown Member of the upper Torreya Formation (Hawthorn Group) in northern Gadsden County contains an early Barstovian land-mammal fauna, has 87 Sr/ 86 Sr age estimates between 14.7 ± 1.5 and 16.6 ± 1.0 Ma, is of reversed magnetic polarity, and probably correlates with Chron C5B-R. These results constrain the age of the Dogtown Member in the study area to between about 15.3 and 15.9 Ma, significantly younger than previously recognized for the upper Torreya Formation. A second fossil locality from the Dogtown Member occurs in the same lithostratigraphic interval but is older, based on vertebrate biochronology. This indicates that the Dogtown Member is time transgressive, that is, younger to the north. Early Hemingfordian land-mammal faunas and an invertebrate fauna correlative with planktonic foraminiferal zones upper N5 and N6 are known from the lower Torreya Formation. One mollusc sample from the Seaboard locality produced a 87 Sr/ 86 Sr age estimate of 18.4 ± 1.0 Ma, consistent with the biochronology. The revised age of the Torreya Formation is late early to early middle Miocene, between about 19 and 15.3 Ma. The Chipola Formation has been interpreted to be late early or middle Miocene (N7 and N8) in age and younger than the Torreya Formation, based on the superposition of the Chipola over the Torreya at the only known stratigraphic contact and apparent biochronologic differences. Four samples from the Chipola Formation at Alum Bluff yield 87 Sr/ 86 Sr age estimates between 18.3 and 18.9 (±1.0) Ma (early Miocene), much older than previous interpretations. A small land-mammal fauna from an overlying unit supports the older age for the Chipola Formation. The discrepancy in ages is probably due to poor biostratigraphic correlation rather than erroneous 87 Sr/ 86 Sr ages. The lithostratigraphic relationship between the Chipola and the Torreya Formations is poorly known from cores only; chronostratigraphic evidence indicates that the Chipola and Torreya Formations overlap in age and may interdigitate, possibly representing transitional marine and nearshore/shelf facies.