MICROFOSSILS WITH MACRO-IMPACT: ANITA HARRIS’S LEGACY IN ALASKAN GEOLOGY

The long and fruitful career of Anita Harris involved studies of conodonts from around the world, but nowhere was the impact of her work more deeply felt, perhaps, than in Alaska. From 1974-2006, her lab processed >8000 samples of Alaskan rocks. About half were productive, and the results of her efforts transformed our understanding of the stratigraphy, depositional settings, paleogeography, and tectonic and thermal histories of Paleozoic-Triassic strata across the state. Conodonts were found in half of the 153 Alaskan 1:250,000 quadrangles, from Dixon Entrance in the south to Barrow in the north.

For many metasedimentary sequences—widely exposed through Alaska—conodonts provide the only firm, often tightly constrained, ages. Penetratively deformed blueschist-, greenschist-, and even amphibolite-facies metamorphic rocks have yielded conodonts—many identifiable to zone (e.g. Nome Complex on Seward Peninsula). Conodonts were particularly useful in understanding metacarbonate rocks (e.g., Baird Group) that crop out in a >700 km-long belt in the Brooks Range. These rocks were thought to be Devonian, based on a few scattered megafossils; conodonts revealed a detailed stratigraphy that extends from early Cambrian into the Carboniferous and accumulated in platform to basinal settings. Ordovician conodonts from northern and interior Alaska include forms with Siberian affinities that supplied early evidence for distant paleogeographic origins of some Alaskan terranes.

The use of conodont color alteration indices (CAIs) as a record of thermal history that Anita invented has become a tool crucial to petroleum and mineral resource research. CAIs of Alaskan conodonts range from 1 (thermally unaltered) to 8 (host strata reached temperatures >600°C). Conodonts from the Lisburne Group, a heterogeneous, partly metamorphosed carbonate unit found in outcrop and subsurface across northern Alaska, have helped us unravel deformational histories, as well as understand the unit’s potential as a petroleum source and reservoir, and as a host of phosphorites and world-class deposits of Zn and Ba (Red Dog District). Anita’s studies of Alaskan conodonts answered an array of geologic questions across a wide range of time and space and continue to provide a basis on which to posit many more queries for generations to come.