COLORADO’S LYKINS FORMATION: AN OVERLOOKED SUCCESSION OF CRINKLED LIMESTONE, MINIATURE MOLLUSCS, AND PERMIAN-TRIASSIC MASS EXTINCTION

The Lykins Formation is notable because it contains a conformable Permian-Triassic boundary succession and because its redbeds yield picturesque soils all along Colorado’s Front Range, from the New Mexico to the Wyoming borders. Yet despite over a century of study, the unit is not widely known and its mass extinction signature has never been studied. The Lykins is obscure even to Coloradans, because only two peer-reviewed papers on the unit have ever been published and because the unit lacks economic value—save for limestone and gypsum which have been sporadically quarried for quicklime and alabaster. The Lykins weathers easily, leading to covered exposures in canyons and valleys, and there are few exposures or drillcores where the upper half of the unit has not been removed by Triassic-Jurassic erosion.

Despite these issues, in places the Lykins is well-exposed and stratigraphically complete. Such exposures reveal a ~330 m thick succession dominated by mudstone, bedded gypsum-anhydrite, and “crinkled” limestone-dolostone, with minor cherty, halitic, and bituminous limestone and rare conglomeratic or lensoidal sandstone. Borehole and drillcore data suggest that the Lykins was deposited along a low relief, gently inclined basin located in an arid to semi-arid area to the paleo-east-southeast of the Ancestral Rockies.

Lykins limestones are stromatolitic, peloidal, and pervasively dolomitized. Often they are capped with evaporate solution breccias and exhibit evidence for early lithification and desiccation. Clastic sediments bear raindrop imprints, polygonal cracks, lamination, and oscillation and ladder ripples, suggesting settling in intermittently emergent, quiescent, or oscillatory conditions. Isopach thicknesses and stacking of evaporates and carbonates around paleotopographic highs suggests the Lykins sea had density-stratified hypersaline waters, and that precipitates formed in barred basin or sabkha settings that intertongued with floodplain and supratidal-intertidal mudflats. Fossils in the unit are rare but are internally consistent with this interpretation because they include abundant microbialites, rare microscopic mollusk coquinas, and possible fish, codiacean algae, foraminifera, and logs.