| Sixtymile Formation | Kwagunt Formation | Galeros Formation | Nankoweap Formation |
| Cardenas Lava
| Dox Formation
| Shinumo Quartzite
| Hakatai Shale
| Bass Limestone |
The Cardenas Lava
The Cardenas Lava is stratigraphically the uppermost and youngest of the Unkar Group of formations in the Grand Canyon Supergroup. The Cardenas Lava conformably contacts and interfingers with the uppermost sandstones of the Dox Formation's Ochoa Point Member. Dox time ended with the onset of a period of volcanic activity producing repetitive basalt flows in the region which compose the overlying Cardenas Lava. The Cardenas outcrops and is exposed only in eastern Grand Canyon, where its thickness is reported to range from about 785 to 985 feet.
The above overviews from the Comanche Point vicinity illustrate exposures of the Cardenas Lava, and the younger Dox and older Nankoweap Formations on the west side of the Colorado in the Tanner Rapids vicinity (D = Dox Formation, C = Cardenas Lava, N = Nankoweap Formation). Outcrops and exposures of Supergroup and Paleozoic formations in the eastern region of Grand Canyon are offset and discontinuous due to extensive faulting that has occurred along the Butte Fault and a pervasive network of branches, spurs and associated faults in the region. Note the isolated, dark curtain of Cardenas Lava outcropping just above one of the nooks in the river with the normally underlying Dox formation adjacent on either side of it. This anomalous feature is due to movement along the Basalt Canyon Fault and Butte Fault bounding on either side where the Cardenas outcrop laterally contacts the Dox. A relative drop of the intervening block of crust between these faults has occurred here.
Normal and reverse movement along Butte Fault is thought to have first dropped the Supergroup by about 5000 feet on the fault's west side prior to its burial by the Paleozoic column. This fault was later reactivated with the block on the west side moving back up about 2700 feet, warping the overlying Paleozoic column and creating the East Kaibab monocline in the process.
The image above left from an overlook atop the Palisades cliffs, illustrates several prominent exposures and outcrops of the Cardenas along the west bank of the Colorado River in the vicinity between Tanner Rapids and Temple Butte. Just upstream and across the Colorado are Lava Butte, Chuar Lava Hill and the Temple Butte Formation. The Butte Fault parallels the Colorado River and lies immediately to the west of Temple Butte as well as Chuar and Kwagunt Buttes and Nankoweap Mesa lining the west side of the river just upstream and north of Temple Butte. Each of these structures and the river bed below them are composed of Paleozoic strata spanning the Tapeats through the Kaibab. The block on the west side of the Butte Fault across from these Paleozoic structures was downfaulted during Precambrian time, preserving the Supergroup formations on that side of the fault.
The image above right details the immediate vicinity of Chuar Lava Hill, a heavily eroded exposure of the Cardinas Lava between the confluences of Carbon Creek and Lava Creek on the west bank of the Colorado River. Underlying and to the left of Lava Hill is pinkish purple Supergroup Dox Sandstone (note its pronounced tilt), with the banded Supergroup Galeros Formation, cut by Carbon Creek Canyon, forming the background. The Palisades Segment of the Butte Fault forks off to the southeast and crosses immediately behind Chuar Lava Hill in this picture, crosses the river just north of Lava Canyon Rapids, runs up Palisades Creek Canyon, across Espejo Butte and then cuts southwards along the edge of the Palisades cliffs to the vicinity of Tanner Canyon. Note the upwarped ribbon of Paleozoic Tapeats Sandstone lying on the north side of the Palisades Segment Fault and immediately adjacent to the top of Chuar Lava Hill.
A thin, discontinuous basalt flow is preserved in the Ochoa Point Member several meters below the Dox/Cardenas contact. The lithology of the uppermost Dox suggests a tidal flat environment and that the region was at or near sea level during the onset of the repetitive eruptions extruding the Cardenas basalts. Local features and mixing at the contact suggest that the basalt overflowed wet, unconsolidated Dox sand and sediments. The Cardenas is characterized by recurring horizons and discontinuous lenses of interbedded sandstone similar to Dox lithology, which suggests the erosion and transport ongoing during Dox time continued to operate and contribute sediments that accumulated throughout the Cardenas during intervals between the eruptions and flows extruding the igneous layers.
The general trend in the region during the accumulation of the Cardenas was one of subsidence. However, during some periods of Cardenas time the accumulation of its volcanic pile in eastern Grand Canyon appears to have exceeded the overall rate of regional subsidence. Some stratigraphic features of the Cardenas, such as occurrences of discontinuous sandstone lenses, suggest intervals above sea level when water flowed over or ponded on its surface.
The Cardenas Lava is unique among the otherwise sedimentary formations in the Grand Canyon Supergroup in that it can be more or less reliably dated using radiometric techniques due to its igneous nature. Radiometric dating by geologists employing the Rubidium-Strontium method places Cardenas time at approximately 1100 million years ago. Dating of various Cardenas samples using the Potassium-Argon method has produced estimated ages ranging from about 780 to 810 million years.
Radiometric dating of the underlying Vishnu Group places its metamorphosis at about 1750 million years ago, so the ages of the sedimentary formations of the Unkar Group must be younger than the Vishnu and older than the Cardenas, which stratigraphically overlies them. Data from studies of paleomagnetic pole positions and polar wandering indicates the Unkar Group was deposited over a period from approximately 1250 million to 1070 million years ago, which correlates with Rb-Sr radiometric dating of the Cardenas for the end of Unkar time. Some geologists theorize an episode of regional heating which may have reset the K-Ar "clock" about 800 million years ago to explain the differences in results between Rb-Sr and K-Ar dating of the Cardenas.
Lower and upper members can be identified in the Cardenas based on their topographic expression. The lower member ranges from about 245 to 295 feet in thickness and erodes to form granular slopes. The olivine rich basalt flows of the lower member are highly weathered, altered and broken, and decompose into nodules typically ranging from several inches to about a foot in size. The thin, discontinuous flows of the lower member are interbedded with sandstone, and the basalt grades into more massive and less altered material towards the top of the member. This lower member has been referred to by some geologists as the "bottle green member". The chemistry, nodular character and glassy texture of the lower member suggest the basalt may have been quenched in brackish or marine water as it extruded.
The upper member of the Cardenas is composed of basalt and basaltic andesite flows with interbedded sandstone that erodes to a cliff forming topology. Near the base of the upper member, about 328 feet above the Dox contact, is a continuous bed of cliff forming, laminated sandstone about 16 feet thick. Directly above this sandstone horizon is a volcanic pile consisting of autoclastic breccia, followed by a fan-jointed unit, a ropey lava and then a lapillite unit at 754 feet above the Dox contact. Features preserved in various flow members of this sequence in the upper member suggest their extrusion and accumulation occurred at a greater rate than the general subsidence ongoing in the area and that they accumulated in subaerial conditions. However, its planar upper surface and a continuous sandstone layer just above the lapillite indicate it subsided below sea level during a period of volcanic quiescence following its accumulation.
Succeeding eruptions and flows during Cardenas time continued to produce further accumulations of basalt and lava of unknown extent. Tectonic activity tilted the Cardenas and underlying Unkar Group formations gently towards the northeast after the Cardenas volcanic activity ceased. Some geologists believe a major displacement occurred along the Butte Fault in eastern Grand Canyon sometime after the cessation of volcanic activity during Cardenas time and prior to the accumulation of the overlying Nankoweap sediments. An unknown amount of the Cardenas Lava was lost to erosion prior to Nankoweap time and its contact with the overlying Nankoweap formation is unconformable. This unconformable contact between the Cardenas and overlaying Nankoweap formation marks the end of Unkar time.