The Cardenas Basalt, also known as either the Cardenas Lava or Cardenas Lavas, is a rock formation that outcrops over an area of about 310 km2 (120 mi2) in the eastern Grand Canyon, Coconino County, Arizona. The lower part of the Cardenas Basalt forms granular talus slopes. Its upper part forms nearly continuous low cliffs that are parallel to the general course of the Colorado River. The most complete, readily accessible, and easily studied exposure of the Cardenas Basalt lies in Basalt Canyon. This is also its type locality.
The Cardenas Basalt is part of the Unkar Group. The Unkar Group is about thick and composed, in ascending order, of the Bass Formation, Hakatai Shale, Shinumo Quartzite, Dox Formation, and Cardenas Basalt. In ascending order, the Cardenas Basalt is overlain by the Nankoweap Formation, about thick; the Chuar Group, about thick; and the Sixtymile Formation, about thick. The Grand Canyon Supergroup, of which the Unkar Group is the lowermost part, overlies deeply eroded granites, gneisses, pegmatites, and schists that comprise Vishnu Basement Rocks.
The Cardenas Basalt has also been called the Rama Formation. However this name, which was originally applied to the dikes and sills intruding strata underlying the Cardenas Basalt has been formally abandoned in the geological literature.
Description
The Cardenas Basalt is about thick and is typically divided into lower and upper units. The lower unit ranges in thickness from and forms low, talus covered slopes. It consists of complexly interbedded, thin, and discontinuous beds of basalt, hyaloclastite, and sandstone. Within the lower part of the Cardenas Basalt, the basaltic lavas are highly fractured and weather into rubble that is about in diameter. The lava within this unit consists of pahoehoe lava flows of olivine-rich basalt. Within the lower part of the Cardenas Basalt, the lava is highly altered and might have been glassy at one time. Near the top of the lower unit the basalt is more massive and less altered. The hyaloclastite is highly altered, and contains secondary chlorite, epidote, talc, and zeolites. Although this unit is highly altered and weathered, many of the primary features are preserved. Thin discontinuous sandstone beds are interbedded with lava flows and hyaloclastite. The brown, maroon, purple sandstones consist of texturally immature, planar-bedded, poorly sorted quartz and feldspar in a matrix of mica and clay. The coarser grains range from medium sand to silt.
The upper unit of the Cardenas Basalt is a series of cliff-forming basaltic and andesitic lava flows that are interbedded with beds of breccia, sandstone, and lapillite. It is about thick and contains four to six, prominent lava flows that range in composition from quartz tholeiite to tholeiitic andesite (icelandite). Some of the lava flows are fan-jointed, ropy, and have porphyritic to aphanitic and vesicular textures. The sandstones within the upper part of the Cardenas Basalt contain eroded fragments of lava and have been baked by overlying lava flows. The lapillite bed ranges in thickness from a few meters to several tens of meters and consists of scoriaceous lapilli, volcanic blocks, and volcanic bombs.
The basaltic and andesitic dikes and sills that occur within the strata underlying the Cardenas Basalt are similar in mineralogy and chemistry to the Cardenas volcanic rocks. This suggests that these intrusive and extrusive rocks are coeval and share a common source. The sills range in thickness from a few tens of meters to as thick as 300 m. The dikes typically are much thinner and locally follow fault planes.
Nature of contacts
The basal contact of the Cardenas Basalt with the underlying Dox Formation is smooth, planar, parallel to bedding and locally interfingering. In places the sandstones of the Dox Formation have small folds and convolutions that are indicative of soft sediment deformation. In addition, in places, the uppermost of the Dox Formation is mildly baked. A thin lava flow occurs within the uppermost part of the Dox Formation. Thus, the contact between the Cardenas Lavas and the Dox Formation is conformable and interfingering. This indicates that sands were still being deposited when the first lavas erupted and that deposition occurred during the transition from the accumulation of Dox Formation to Cardenas Basalt.