Introduction to the
Mojave National Preserve
& Ancient Lakes
Weathering & Erosion
Faults & Active Tectonics
Pediments & Alluvial Fans
& Older Surfaces
Sand Dunes & Dust
3D Geology Tour
Carbonate Rocks and Associated Landforms
In the Mojave National Preserve, carbonate sedimentary rocks of Proterozoic and Paleozoic age
(consisting of limestone and dolostone) crop out throughout the Clark Mountains, the Mescal
Mountains, in the northern Ivanpah Range, in the central portion of Providence Mountains,
and elsewhere. Carbonate rocks originally form from limey sediments consisting of the calcareous
skeletal remains of algae and invertebrate shell material or precipitates directly from agitated,
warm seawater (as on a shallow continental shelf in a warm climate). Most ancient limestones
formed from planktonic algae, but in late Paleozoic time coralline reefs became significant
producers of carbonate sediments. Limestone consists dominantly of the mineral Calcite-CaCO3,
whereas dolostone consists dominantly of the mineral dolomite-CaMg(CO3)2. Dolomite is typically
a secondary mineral replacement of original calcite material. Ancient carbonate rocks like those
in the Mojave region tend to be enriched in dolomite. Below are examples of common fossiliferous
carbonate rocks of Paleozoic age from the Mojave region.
|Oncolites (algal limestone balls) float in a carbonate-mud matrix
in the Cambrian-age Chambless Formation. This easy-to-recognize oncolite-bearing
limestone formation crops out in many areas throughout the Mojave
National Preserve region. These nearly spheroidal oncolites formed
by algae and/or cyanobacterial growth in shallow warm marine waters
of a carbonate platform environment.
|A boulder of a fossiliferous limestone displays stromatoporoids
and corals of early Devonian age (Sultan Formation). Boulders like
this one are not uncommon in alluvial fans downstream from the Paleozoic
age sedimentary rock belt exposed in the Providence Range.
|An expansive Joshua-tree forest covers a pediment surface (along
Cima Road). In the distance, steeply dipping and folded sedimentary
rocks (mostly limestone and dolomite) of late Proterozoic and Paleozoic
age crop out throughout the Mescal Range.
In contrast to other types of rocks, carbonate rocks tend to be fairly
resistant to erosion in arid climate conditions. The ancient carbonate
rocks in the Mojave region are typically both dense and brittle and tends
to be heavily fractured at the surface. At depth, fractures in carbonate
tend to heal over time as the rock gradually flows under extreme pressure,
and as calcite and other minerals precipitate in crevasses over time.
Throughout the desert southwest, deep canyons carved into carbonate rock
display collapse breccia, which are massive surficial deposits that consist
of broken fragments of limestone and dolostone tightly cemented in a carbonate
||A small cavern occurs in brecciated carbonate rocks that crop out
along an unnamed wash draining from the western Providence Mountains
(8 miles directly east of Kelso Depot). "Collapse breccias" like this
are prevalent in the Bonanza King Formation. This formation is nearly
300 meters thick and consists of algal limestone and dolomite of Middle
Cambrian age (around 550 million years). The age of the "collapse"
is unresolved, but may actually be an active physical-chemical process
that affects large carbonate units in the desert surface environment.
They may actively be "flowing" down slope under the force of gravity.
This breccia is probably an early Quaternary-age landslide deposit
that has been reconsolidated (cemented) by groundwater interaction
with the carbonate rock.
Carbonate rocks dissolve in freshwater, with calcite being more soluble
than dolomite. With each precipitation event, traces of carbonate material
will dissolve and migrate with flowing water. Dissolution occurs along
fractures in the subsurface producing caverns. As water evaporates at
the surface, calcite will precipitate again, cementing sediments on alluvial
fans to form a durable calcareous crust (called caliche). Calcite
is a major component of playa mud deposits. Varieties of freshwater limestone
deposits called tufa and travertine form around springs and in former
wave-influenced lake shore zones. In caverns, travertine deposits are
called speleothems (which include stalagmites, stalactites, columns, flowstone,
and other features).
|Mitchell Caverns in the Providence Mountains State Recreation Area has
been developed for commercial visitation. The cavern formed in limestone
of Late Paleozoic age (Pennsylvanian- and Permian-age Bird Springs
Formation). The cavern formed long ago when the bedrock was constantly
exposed to groundwater, allowing dissolution of the limestone to occur.
Today, the cavern is high and dry; it is more than 500 meters above
the valley floor and a constant supply of groundwater.
|Travertine tapestries, flowstone, and other speleothems gradually
form where groundwater enters the cavern and evaporates, leaving behind
calcium carbonate. Features like this one probably took many thousands
of years to form. Only a small percentage of the speleothems in Mitchell
Caverns are actively forming.
In the Mojave region, alluvial deposits derived from areas with carbonate
bedrock tend to consist of blocky, unevenly sorted sediments. In many
areas chert layers and metasandstone layers occur interbedded within the
bedrock. These more siliceous materials tend to be more resistant to both
mechanical and chemical weathering forces, and as a result, alluvial surfaces
and sediments down slope from carbonate rock source areas tend to be enriched
in these associated siliceous materials. Carbonate mountains are the highest
and steepest without exception. These areas are prone to stronger flood
forces, bigger canyons, more precipitation, coarser fans, steeper fans,
and hense, greater risk for debris flow activity.