Sequence stratigraphy and depositional environments of the Harding Sandstone, central Colorado : implications for the habitat of early fish

Sixteen outcrops throughout the Caon City Embayment and surrounding areas in Colorado were stratigraphically described to conclude that two ancient genera of fish, Astraspis desiderata and Eriptychius americanus, inhabited a low wave energy microtidal shallow marine environment. Debated for over a century, the habitat of these fish has been previously interpreted as fluvial, estuarine and marine. The facies interpretation and sequence stratigraphic analysis presented here provide new understanding to the ongoing debate. The Harding is interpreted here as mostly shoreface facies interrupted with an interval of lagoonal associated facies in the Caon City area. Low wave energy conditions near the shore allowed extensive burrowing by organisms in the sediment that resulted in the pervasive bioturbation and lack of sedimentary structures. Fish remains are most commonly preserved on flooding surfaces either overlying or underlying shoreface facies and are associated with minor condensed intervals.