THE GEOLOGY OF THE CERRILLOS HILLS
by Stephen R. Maynard CONSULTING GEOLOGIST
Contracted to the NM BUREAU OF MINES AND MINERAL RESOURCES for QUADRANGLE-SCALE GEOLOGIC MAPPING
LANDSCAPE AND TOPOGRAPHY
The Cerrillos Hills is a group of low mountains that covers an area about 18 square miles [46.6 square kilometers] north of the village of Cerrillos, in Santa Fe County, New Mexico. The Cerrillos Hills' lowest elevation is 5600 ft [1,706.9 meters], along the Galisteo Creek. Most of the Cerrillos Hills is hilly terrain cut by locally steep-sided gulches and arroyos. The hilltops of most of the land range from 6000 to 6200 feet [1,828.8 to 1,889.8 meters] above sea level. Several prominent hills (Cerro Cosena, Grand Central Mountain, Cerro Bonanza, Lucera Hill, and Achavica Mountain) rise to summits over 6900 ft [2,103.1 meters]. The Santa Fe plateau, on the on the north side of the Cerrillos Hills, lies at about 6300 ft [1,920.2 meters] above sea level.
Galisteo Creek borders the southern flank of the Cerrillos Hills and flows east to west, to the Rio Grande. San Marcos Arroyo cuts through the southeastern part of the Cerrillos Hills and joins Galisteo Creek at the village of Cerrillos. Both streams flow intermittently, though Galisteo Creek usually has at least a trickle of water. Both streams are capable of flash floods as well. Other gulches and arroyos draining the Cerrillos Hills are ephemeral.
The Cerrillos Hills are covered by the Madrid, Picture Rock, Tetilla Peak, and Turquoise Hill U.S. Geological Survey 7.5-minute topographic maps.
REGIONAL GEOLOGIC HISTORY
Precambrian Era - (4500- 570 million years ago)
The Precambrian Era is the first part of Earth's history, beginning with the formation of the Earth, about 4,500 million years ago and ending with the advent of complex life forms 570 million years ago. In north-central New Mexico the Precambrian Era is represented by 1,600- to 1,700-million years-old metamorphic rocks and 1,400-million years-old granite in the Sandia and Sangre de Cristo Mountains. Precambrian rocks are not exposed in the Cerrillos Hills, except as isolated boulders of granite found as inclusions in the igneous rocks. These inclusions were likely carried up several thousand feet by the molten rock to their present positions.
There is no record of the Earth's history in New Mexico from about 1400 million years ago to the Mississippian Period, about 330 million years ago. Geologists surmise that at least for the latter part of that time, the land surface was above sea level and was probably fairly level.
Paleozoic and Mesozoic Eras (570 to 65 million years ago)
The Paleozoic Era began 570 million years ago. As mentioned before, there is no record of the early part of the Paleozoic Era in the Cerrillos Hills area. During the Mississippian and Pennsylvanian Periods, 330 to 280 million years ago, north-central New Mexico lay under a shallow sea. The limestone that caps the Sandia Mountains and is found in the upper Pecos region of the Sangre de Cristo Mountains was deposited during the Pennsylvanian Period.
During the Permian Period, at the end of the Paleozoic Era, and the Triassic Period, at the beginning of the Mesozoic Era, 280 to 195 million years ago, the seas receded from north-central New Mexico. Sandstone and mudstone was deposited on broad flood plains. The earliest reptiles arose during the Permian Period and dinosaurs appeared during the Triassic Period. Permian and Triassic rocks of the region are typically reddish to maroon and crop out over a broad area of New Mexico.
The region stayed above sea level during the Jurassic Period and the early Cretaceous Period (195 to 100 million years ago). Sand and mud, partly eroded from volcanoes in southwestern Arizona, California, and Nevada, were deposited on river flood plains during the Jurassic Period, as well as limestone and gypsum in a shallow lake hundreds of miles wide. Morrison Formation sediments of the late Jurassic Period are the oldest sedimentary rocks that crop out in the Cerrillos Hills.
There was no further deposition of sediments from the end of the Jurassic Period, 140 million years ago, until the beginning of the late Cretaceous Period, 100 million years ago. The land continued to be above sea level during this time.
The sea returned to north-central New Mexico in the late Cretaceous Period. Sandstone and shale of the Dakota Formation, and black to gray shale, and lesser sandstone and limestone of the 3,000-foot [914.4 meters] thick Mancos Shale, were deposited in a shallow sea. This shallow sea extended from the Gulf of Mexico to the Arctic Ocean and covered most of the Rocky Mountain and High Plains states and provinces. Sandstone of the Dakota Formation, often displaying its characteristic fossil worm burrows and traces, crops out in several locations around the Cerrillos Hills. The black or gray shale of the Mancos Shale is widely exposed in the Cerrillos Hills. The Mancos Shale forms most of the lowlands along County Road 52 (the Waldo Road) west of Cerrillos village. Ammonite fossils are often found in the Mancos Shale.
The sea retreated again from the Cerrillos Hills area in the late Cretaceous Period and was replaced by an environment of sluggish streams and swamps separated from the sea by a broad sandy beach. The sandstones and shales deposited during that time are known as the Mesa Verde Group, which extends over much of the Four Corners region. The large coal deposits of the Colorado Plateau, and the smaller ones of the Madrid area, occur in Mesa Verde Group rocks. The Mesa Verde Group, with a handful of small coal prospects, crops out in the southeastern part of the Cerrillos Hills.
During the latest part of the Cretaceous Period and the beginning of the Tertiary Period of the Cenozoic Era, western North America experienced a mountain-building episode known as the Laramide Orogeny. (orogeny = mountain-building). The modern Rocky Mountains developed during this time. The entire area was eroded, though the Cerrillos Hills area was not tilted.
The end of the Cretaceous Period and the beginning of the Tertiary Period, about 65 million years ago, is marked by the disappearance of the dinosaurs and many other species, and the emergence of mammals and flowering plants. It is believed by many geologists that this extinction was caused by a catastrophic event, probably by the collision of a large meteorite with the earth. Evidence for this event cannot be found in the Cerrillos Hills area.
Cenozoic Era (65 million years ago to present)
During the earliest part of the Tertiary (65 million to about 35 million years ago) a broad intermontane basin extending from Lamy to Placitas filled with sedimentary material derived from nearby mountains. These sedimentary rocks are sandstone and mudstone of the Diamond Tail and Galisteo Formations. Sandstone of the Diamond Tail Formation forms the buff-colored cliffs along NM-14 between Madrid and Cerrillos. The Diamond Tail and Galisteo Formations are beautifully exposed on the eastern side of the Cerrillos Hills, where they have been tilted to vertical. The vivid red mudstone between the sandstone layers belong to the Galisteo Formation.
The Cerrillos Hills began to take their present form with the invasion of the sedimentary rocks by molten rock (magma) 34 to 30 million years ago. The formation of the Cerrillos Hills Igneous Complex resulted in four important features. The first is that magma replaced much of the sedimentary rocks. Second, the remaining sediments were pushed up around the edges of the intrusive complex. Third, the deposits of silver, lead, zinc, and copper formed as the last stage of the crystallization of the igneous rocks. Fourth, volcanic rocks, in the form of lavas and pyroclastic flows, were erupted from a source thought to lie on the northeastern side of the Cerrillos Hills. The volcanic rocks that erupted from the Cerrillos Hills and the Ortiz Mountains make up the Espinaso Formation. The Espinaso Formation is exposed in the northeastern part of the Cerrillos Hills and makes up the low hills to the east of NM-14, north of Galisteo Creek.
Cerrillos Hills Igneous Complex
The Cerrillos Hills Igneous Complex is considered by geologists to be part of the Ortiz Porphyry Belt, a 25-mile [40.2 kilometer] long, north-south trending group of igneous rocks that begins at South Mountain in the south, and ends at La Ciénega in the north. Throughout the Ortiz Porphyry Belt, the igneous rocks have invaded sedimentary rocks ranging from the Mississippian Period to the Tertiary Period. In the San Pedro Mountains, the Ortiz Mountains, and in the Cerrillos Hills, the igneous rocks have a direct relationship to the ore deposits found there.
In the Cerrillos Hills, igneous rocks are both plutonic (or intrusive) and volcanic (extrusive). The intrusive rocks can be divided into two main groups. The first group is about 34 million years old and is composed of quartz-bearing andesite-latite porphyry. The andesite-latite porphyry generally invaded the sedimentary rocks in sheets parallel to the sedimentary rocks' layering. The individual sheets are called sills. The sills were probably fed by vertical fractures. These vertical fractures filled with igneous rock are called dikes. A series of stacked sills connected by dikes is called a Christmas-tree laccolith. It is believed that such a laccolithic structure existed in the Cerrillos Hills, though its original form has been mostly destroyed by younger plutonic rocks, and later tilting and erosion.
Around 30 million years ago a series of quartz-poor latite and monzonite bodies (dikes and stocks) intruded the Cerrillos laccolith. These intrusions pushed the country rock up and tilted it away from the centers of the intrusive bodies. The monzonite stocks that form the highest hills in the Cerrillos Hills were formed at this time. A small, as yet unmined, copper deposit formed in the central part of the Cerrillos Hills at this time. Veins containing lead, zinc, and silver formed in northeast-trending fractures shortly afterwards (within a million years or so!)
From about 30 million years to about 3 million years ago, a series of linked basins formed (the Rio Grande Rift) and filled with sediment. The Cerrillos Hills area was eroded and the broad plain of the Santa Fe Plateau formed. Geologists call this feature a peneplain. The peneplain appears to have extended over the Cerrillos Hills, with the highest hills, e.g. Cerro Cosena, Grand Central Mountain, Cerro Bonanza, Lucera Hill, and Achavica Mountain, left standing above its sloping surface. Similar peneplains are found over much of New Mexico. One of the most striking is the Ortiz Surface, which flanks the Ortiz Mountains on all sides and is easily viewed from the Cerrillos Hills.
During the period 2.8 million to 1.5 million years ago, during the time periods known as the Pliocene and Pleistocene Epochs, streams draining the Sangre de Cristo Mountains deposited sand and gravel on the Santa Fe Plateau. This deposit is known as the Ancha Formation. The Ancha Formation covers the Plateau with up to 40 ft. of sand and gravel. It can be seen in the eastern and northern parts of the Cerrillos Hills. Wolf Road, in the eastern part of the Cerrillos Hills, runs along a ridge capped with Ancha Formation gravel. A similar deposit called the Tuerto Gravel caps the Ortiz Surface that flanks the Ortiz Mountains.
As the Rio Grande Valley deepened in the last 1.5 million years, so did Galisteo Creek. Tributary arroyos and gulches cut down through the rock as well, first stripping away the Ancha Formation and Tuerto Gravel deposits, leaving them perched on mesa tops. Both Galisteo Creek and San Marcos Arroyo have significant terrace deposits that record times in recent geologic history when the streams were tens of feet higher than they are now and left stream-worn gravel on benches above their present courses.