Carol A. Hill

Karst piracy: A mechanism for integrating the Colorado River across the Kaibab uplift, Grand Canyon, Arizona, USA

Age, isotopic, and detrital zircon data on the Hualapai Limestone Member and Muddy Creek Formation (western United States) constrain the time of the first arrival of the Colorado River on the west side of the Grand Canyon to ca. 6–5 Ma. We propose a karst piracy mechanism, along with a 17–6 Ma western paleo–Grand Canyon, as an alternative explanation for how the Colorado River became integrated across the Kaibab uplift and for the progressive upsection decrease in δ 18 O and 87 Sr/ 86 Sr values of the Hualapai Limestone Member. An earlier Laramide paleocanyon, along which this western paleocanyon followed, can also perhaps explain why no clastic delta exists in the Grand Wash trough. Karst piracy is a type of stream piracy where a subterranean drainage connection is made under a topographic divide. The process of karst piracy proceeds through five main stages: (1) establishment of a gradient across a topographic divide due to headward erosion into the low side of the divide, (2) leakage in soluble rock along the steepest gradient, (3) expansion of the leakage route into a cave passage that is able to carry a significant volume of water under the divide, (4) stoping and collapse of rock above the underground river, eventually forming a narrow gorge, and (5) widening of the gorge into a canyon. A karst piracy model is proposed here for the Kaibab uplift area that takes into account the structure and hydrology of that area. Other examples of karst piracy operating around the world support our proposition for integrating the Colorado River across the Kaibab uplift in the Grand Canyon.

Sulfuric Acid Caves

Most caves owe their origin to carbonic acid generated in the soil. In contrast, sulfuric acid caves are produced by the oxidation of sulfides beneath the surface. Although sulfuric acid caves are relatively few, they include some large and well-known examples, such as Carlsbad Cavern, New Mexico. They also provide evidence for a variety of deep-seated processes that are important to petroleum geology, ore geology, tectonic history, and the nascent field of karst geomicrobiology.

Huntite flowstone in carlsbad caverns, new Mexico.

Huntite flowstone has recently been discovered in Carlsbad Caverns. This flowstone occurs as a thin, white layer of microcrystals (approximately 1 to 60 micrometers in diameter) which appears buckled and crinkled. The huntite is believed to be precipitating directly from magnesium-rich solutions rather than forming by alteration of preexisting minerals.

Constraints on a Late Cretaceous uplift, denudation, and incision of the Grand Canyon region, southwestern Colorado Plateau, USA, from U‐Pb dating of lacustrine limestone

The uplift and denudation of the Colorado Plateau is important in reconstructing the geomorphic and tectonic evolution of western North America. A Late Cretaceous (64 ± 2 Ma) U-Pb age for the Long Point limestone on the Coconino Plateau, which overlies a regional erosional surface developed on Permo-Triassic formations, supports unroofing of the Coconino Plateau part of Grand Canyon by that time. U-Pb analyses of three separate outcrops of this limestone gave ages of 64.0 ± 0.7, 60.5 ± 4.6, and 66.3 ± 3.9 Ma, which dates are older than a fossil-based, early Eocene age. Samples of the Long Point limestone were dated using the isotope dilution isochron method on well-preserved carbonates having high-uranium and low-lead concentrations. Our U-Pb ages on the Long Point limestone place important constraints on the (1) time of tectonic uplift of the southwestern Colorado Plateau and Kaibab arch, (2) time of denudation of the Coconino Plateau, and (3) Late Cretaceous models of paleocanyon incision west of, or across, the Kaibab arch. We propose that the age of the Long Point limestone, interbedded within the Music Mountain Formation in the Long Point area, represents a period of regional aggradation and a time of drainage blockage northward and eastward across the Kaibab arch, with possible diversion of northward drainage on the Coconino Plateau westward around the arch via a Laramide paleo-Grand Canyon.

Response to Comments on the “Age and Evolution of the Grand Canyon Revealed by U-Pb Dating of Water Table–Type Speleothems”

Pederson et al. and Pearthree et al. offer critical comments on our study of the age and evolution of the Grand Canyon. Both sets of authors question our use of incision rates from two sample sites located outside the canyon and present alternative interpretations of our data. As we explain, even without the sites in question, our data support a “precursor” western Grand Canyon older than 6 million years.

Hypogene Speleogenesis in the Guadalupe Mountains, New Mexico and Texas, USA

The Guadalupe Mountains consist of an uplift of Permian carbonate shelf deposits in a semiarid landscape. A variety of speleogenetic processes, mostly hypogene, have made them one of the world’s best-known cave regions. The most notable caves are Carlsbad Cavern, which contains the largest known cave room in the USA, and Lechuguilla Cave, now the world’s 7th longest. Because the caves are no longer active, there was early confusion about their origin. This was resolved when long-dormant sulfuric acid processes were recognized, with H2S supplied by nearby oil fields. Potassium-argon dating of the by-product mineral alunite in the Guadalupes indicates speleogenetic ages from 12 to 4 million years, decreasing with lower elevation. Caves show abundant evidence for subaerial corrosion, both by sulfuric acid and carbonic acid in water films. Many seemingly phreatic features have resulted from this subaerial process. Microbial alteration of bedrock has contributed to weathering. There is evidence that isolated caves of greater age, lined by large scalenohedral calcite, were formed by supercritical CO2 in deep thermal water.

A Conceptual Model for Hypogene Speleogenesis in Grand Canyon, Arizona

Although Grand Canyon hosts exceptionally well‐formed vadose caves and exemplary paleokarst, it also contains hypogene caves that provide important information on the canyon’s age and evolution. These caves exhibit speleogenetic materials, passage morphologies, and locations near the top of the Mississippian Redwall Limestone that represent major hypogene dissolution phases. Cave origin resulted from CO2 and H2S in solutions that upwelled from depth and mixed with Redwall–Muav aquifer water. Lack of abundant speleogenetic gypsum suggests that CO2 was the primary solutional agent, while upwelling H2S likely played only a minor role. Each phase of hypogene speleogenesis in our model encompasses the following sub‐events, from deep to shallow: (1) dissolution of cave passages 500 ± 250 m below the water table or potentiometric surface, sometimes with Fe‐ and Mn‐oxide by-products; (2) deposition of calcite spar linings (~50–100 m below the water table); (3) deposition of calcite mammillary coatings (1–20 m below the water table); (4) deposition of calcite folia at the water table; and (5) deposition of gypsum rinds a few meters above the water table. Controls on the amount of cave dissolution and speleogenetic by-products probably include regional water table fluctuations during the Miocene and Pliocene, in combination with magmatic/tectonic pulses that pumped CO2 and H2S from below. The complete cycle of Grand Canyon hypogene speleogenesis includes a largely dissolutional phase under confined conditions and a later (mostly by-product) phase taking place under unconfined conditions. The process terminates when the water table descends below the cave.