Preliminary investigation of the origin of the Riley travertine, Socorro County, New Mexico
— James M. Barker

The Riley travertine is a secondary carbonate of complex origin deposited in the upper Santa Fe Group, probably between one and three million years ago. It is the result of lateral groundwater flow from Paleozoic carbonate aquifers and associated bajadas. This flow was both surface and subsurface and produced carbonate deposits of complex morphology with spring, lacustrine, reworked and pervasively or dis-placively cemented facies. The unit is composed of nonpedogenic secondary carbonate deposited on and within existing sediments and pre- Santa Fe rocks; consequently, it is somewhat diachronous. It corresponds to a nonpedogenic groundwater calcrete with valley calcrete predominating and cienega or other calcrete locally abundant.

The groundwater was initially charged with carbonate in peripheral Paleozoic limestone strata or bajadas derived from them. The dominant recharge area was either the west slope of the Ladron Mountains, where uplift provided increased precipitation and greater hydraulic head, or northern and northwestern source areas on the Lucero uplift and possibly the San Juan basin. Groundwater flowed toward the site of north mesa from the west, north, and east then turned south along the valley paleodrainage axis and flowed southeast through the area now occupied by south mesa. The flow merged with a west-to-east drainage which then passed north of San Lorenzo Canyon and probably entered the ancestral Rio Grande. The carbonate-charged waters flowed on and in pre-Tertiary basement rocks and then across Santa Fe sediments at the foot of the slopes and on or under the valley floor. Some water was debouched from springs with extensive subsurface flow below them or was present as episodic surface flows, although the bulk of the carbonate charged water stayed below the surface in the Santa Fe sediments.

The carbonate-charged waters pervasively cemented the host Santa Fe sediments and spread outward and downstream from the source areas. As transmissive zones in the Santa Fe plugged with carbonate cement, a stronger lateral flow developed above these newly formed aquicludes and produced localized laminar deposition related to transmissivity and prior calcite deposition patterns. Contemporaneous surface ponding and spring activity yielded localized lacustrine or algal limestone and travertine. The various types of carbonate accumulation on alluvial fans (Lattman, 1973) were developed locally as shown by the calcite-cemented breccias and conglomerates, some with reworked Riley travertine clasts in them. The massive portions with localized crude laminations, which increasingly dominate the Riley travertine downflow, were precipitated from laterally flowing subsurface groundwaters. The groundwater moved pervasively into some sections and less so into others, where flow pulses over localized plugged zones yielded discontinuous laminations. The bed was thickened by displacive calcite, a process which may yield lamination and floating grains. Matrix material is diagenetically altered and commonly removed during calcite diagenesis, thus increasing the calcite content of the deposit. The processes that formed the Riley travertine seem to be fundamental ones. The Riley travertine, while anomalously large, must represent a type of deposit present elsewhere in the arid southwestern United States and other arid regions of the world. Many secondary calcite deposits formerly thought to be pedogenic are probably nonpedogenic, although differentiating them is often very difficult without detailed study. Further investigation of the interrelations among secondary calcite depositional processes is needed for a clearer understanding of the relationship be-tween pedogenic and nonpedogenic deposits.