PALEO-SPRING DEPOSITS AS RECORDS OF THE BALANCE BETWEEN REGIONAL FLOW IN FAULTS AND LOCAL RECHARGE: A CASE STUDY FROM THE TECOPA BASIN, CA

Past spring discharge (70 to 285 ka) in the Tecopa Basin, CA, is recorded in tufa deposits (mounds, pools, cemented ledges, and rare feeder veins) intercalated with pluvial Lake Tecopa sediments and younger alluvial fan deposits. Existing evidence indicates that the springs discharged subaerially, rather than interacting with evaporated lake water. Tufas are distributed in a linear NW-SE trend along the foot of the Resting Spring Range, and are clearly associated with a range-bounding normal fault (part of the Ash Meadows fault zone).

¹⁸O/¹⁶O ratios in feeder veins indicate that spring discharge represented a lesser mixture of regional discharge upwelling along the fault that mixed with a larger proportion of range-front and alluvial fan recharge that was dammed against fine-grained sediments in the hanging wall. ¹³C/¹²C ratios indicate that the inorganic carbon load was dominated by a high TDS regional component. A review of the isotopic composition of spring deposits in the Death Valley region suggests that a similar water balance was important at many paleo-discharge sites. Two plausible flowpaths exist for the regional component. The first was via interbasin flow in carbonate rocks from the Spring Mountains, beneath Pahrump Valley, the Nopah Range, Chicago Valley, and the Resting Spring Range. The second is from Ash Meadows southward along the damage zone of Ash Meadows fault. Sr- and U-series isotopes indicate that siliciclastic rocks, normally considered a regional aquitard, interacted strongly with regional waters. This suggests that the second flow path was much more likely, as fracture-induced permeability in a damage zone is better suited to provide intimate contact with siliciclastic units. This runs counter to the prevailing model for modern discharge in the Tecopa Basin, which calls upon interbasin flow, and is yet another example that the concept of regional interbasin transfer through carbonate rocks has been over applied.