ASYNCHRONOUS DEPOSITION OF THE OGALLALA FORMATION ON THE NORTH AMERICAN HIGH PLAINS FROM DETRITAL ZIRCON U-PB MAXIMUM DEPOSITIONAL AGES

The Ogallala Formation represents the primary water-bearing unit of the High Plains Aquifer in central North America. It consists of a heterogeneous mix of fluvial and eolian sediments, of which the deposition is thought to span the middle to late Miocene based primarily on North American Land Mammal Ages and U-Pb ages from interbedded volcanic ashes. Here we present new detrital zircon U-Pb ages from throughout the High Plains to further constrain regional chronostratigraphy.

We collected 24 representative samples of medium-to-fine-grained fluvial sandstones from 13 well-exposed outcrops across Texas, New Mexico, Kansas, and Nebraska. The sections we examined collectively expose the whole formation and focus on several initial deposits above the basal unconformity. Detrital zircon grains were recovered through standard mineral separation processes and analyzed for U-Pb ages by LA-ICP-MS at the Arizona LaserChron Center. We analyzed a target group of 600 zircon grains per sample to recover as many grains of Neogene age as possible, which we use to estimate Maximum Depositional Ages (MDAs).

For the northern High Plains, our data supports previous research that indicates the Ash Hollow Formation of the upper Ogallala was deposited between ~12.8–8.1 Ma in Nebraska. We also report an older MDA of 21.85±0.30 Ma from the basal Ogallala in this region. In the central High Plains of Kansas, we obtained MDAs of 11.20±1.10 Ma and 8.46±0.22 Ma, which are in accordance with previous reported ages of syndepositional volcanic ashes in this region. In the southern High Plains, we acquired three MDAs of late Oligocene to early Miocene age from the basal Ogallala in Texas: 21.87±2.81 Ma, 24.12±0.43 Ma, and 31.85±5.75 Ma. In spite of the low abundance of Cenozoic zircon grains in this region, the older MDAs suggest that these deposits represent the earliest accumulation of Ogallala sediments on the High Plains.

In addition to the MDAs, we will use the detrital zircon age distribution for each sample to reconstruct the timing and evolution of the fluvial systems that were responsible for Ogallala deposition as part of a broader examination of this economically important Oligocene-Miocene source-to-sink system in central North America.