UNDERSTANDING RELATIONSHIPS BETWEEN THE ARKANSAS RIVER AND RIFT RELATED FAULTING IN SOUTHERN COLORADO

To test the hypothesis that the modern form of the Arkansas River (AR), in the southern Rocky Mountains of Colorado, is relatively young (Pliocene-Pleistocene) and that a large stream capture event, triggered by uplift of the Poncha Block as an accommodation zone in the RGR, diverted a south-flowing paleo-AR eastward, we combine stratigraphic, sedimentary, and thermochrologic analyses.

We measured three sections of the Miocene to Pliocene(?) Dry Union formation in the uplifted Poncha Block (two in the west and one in the center). Clast counts within the sections show that the two western sections had generally coarser grain sizes with more volcanic compositions indicating a proximal source, possibly from nearby peaks within the Poncha Block. The NW section appears to capture an alluvial fan environment with coarse sand to large boulder conglomerates. The conglomerates are dominantly clast supported and showed multiple possible regressive and transgressive sequences that were evidenced by a coarsening upwards and fining upwards in grain size respectively. The SW section shows a fining upward, possible transgressive, sequence and weak imbrication suggests a NW flow direction. The central section displays grain sizes significantly finer than the other sections, ranging from siltstone to pebble conglomerates. Several layers show a significant amount of organic material, as well as root casts. These characteristics, S-SE paleocurrent indicators, and small pockets of cross beds indicating flow in the opposite direction imply a more fluvial or flood plain depositional environment. Although clast counts at the base of the section show similar compositions of to those in the west, the Dry Union fm. in the central part of the Poncha block is noticeably different.

Ar-Ar data from tephra cut by the Poncha Springs Fault (PSF) indicate that exhumation of the Poncha Block is <8Ma (Hubbard et al., 2001). Anticipated results from apatite(U-Th-Sm)/He samples collected along the PSF will provide more information about the timing and growth of the PSF. Determining uplift timing from thermochronology, and sediment sources for the Dry Union fm. will help track the evolution of the paleo-AR and document development of the Rio Grande Rift.