In-Cave and Surface Geophysics to Detect a “Lost” River in the Upper Levels of the Mammoth Cave System

In early 1960, explorers accessed a significant underground river through a crawlspace beneath a ledge in Swinnerton Avenue southeast of the Duck-Under. However, later expeditions failed to find this crawlspace. Instead, the level of sediment in the passage is now generally at or above the rock ledge, leaving no openings to lower level passages other than the Duck-Under itself. Apparently recent organic material (leaves, twigs, etc.) observed in passages just below the Duck-Under may be related to open channel fl ow from storm events which could theoretically provide local sediment transport. Therefore we have used in-cave spontaneous potential (SP), ground penetrating radar (GPR), and acoustic profiling, as well as surface mise-a-la-masse resistivity profiling, in an attempt to locate the river itself rather than the missing crawlway. Incave dye studies and additional geophysical profiling are needed to work out the detailed 3-D hydraulics of this region of the cave system

Provenance of Cretaceous through Eocene strata of the Four Corners region: Insights from detrital zircons in the San Juan Basin, New Mexico and Colorado

Cretaceous through Eocene strata of the Four Corners region provide an excellent record of changes in sediment provenance from Sevier thin-skinned thrusting through the formation of Laramide block uplifts and intra-foreland basins. During the ca. 125–50 Ma timespan, the San Juan Basin was flanked by the Sevier thrust belt to the west, the Mogollon highlands rift shoulder to the southwest, and was influenced by (ca. 75–50 Ma) Laramide tectonism, ultimately preserving a >6000 ft (>2000 m) sequence of continental, marginal-marine, and offshore marine sediments. In order to decipher the influences of these tectonic features on sediment delivery to the area, we evaluated 3228 U-Pb laser analyses from 32 detrital-zircon samples from across the entire San Juan Basin, of which 1520 analyses from 16 samples are newly reported herein. The detrital-zircon results indicate four stratigraphic intervals with internally consistent age peaks: (1) Lower Cretaceous Burro Canyon Formation, (2) Turonian (93.9–89.8 Ma) Gallup Sandstone through Campanian (83.6–72.1 Ma) Lewis Shale, (3) Campanian Pictured Cliffs Sandstone through Campanian Fruitland Formation, and (4) Campanian Kirtland Sandstone through Lower Eocene (56.0–47.8 Ma) San Jose Formation. Statistical analysis of the detrital-zircon results, in conjunction with paleocurrent data, reveals three distinct changes in sediment provenance. The first transition, between the Burro Canyon Formation and the Gallup Sandstone, reflects a change from predominantly reworked sediment from the Sevier thrust front, including uplifted Paleozoic sediments and Mesozoic eolian sandstones, to a mixed signature indicating both Sevier and Mogollon derivation. Deposition of the Pictured Cliffs Sandstone at ca. 75 Ma marks the beginning of the second transition and is indicated by the spate of near-depositional-age zircons, likely derived from the Laramide porphyry copper province of southern Arizona and southwestern New Mexico. Paleoflow indicators suggest the third change in provenance was complete by 65 Ma as recorded by the deposition of the Paleocene Ojo Alamo Sandstone. However, our new U-Pb detrital-zircon results indicate this transition initiated ∼8 m.y. earlier during deposition of the Campanian Kirtland Formation beginning ca. 73 Ma. This final change in provenance is interpreted to reflect the unroofing of surrounding Laramide basement blocks and a switch to local derivation. At this time, sediment entering the San Juan Basin was largely being generated from the nearby San Juan Mountains to the north-northwest, including uplift associated with early phases of Colorado mineral belt magmatism. Thus, the detrital-zircon spectra in the San Juan Basin document the transition from initial reworking of the Paleozoic and Mesozoic cratonal blanket to unroofing of distant basement-cored uplifts and Laramide plutonic rocks, then to more local Laramide uplifts.National Science Foundation (NSF grant EAR-1649254