Ambos Nogales Geology

 

The Geologic time scale

Before covering this subject, it is necessary to know that the History of Geology has been divided in the following ERAS and PERIODS. This scale will be useful for our understanding of the explanation that follows.

ERA	TIME	PERIOD	TIME
PRECAMBRIAN	Between 4,500 and 570 million years ago
PALEOZOIC	Between 570 to 225 million years ago
MESOZOIC	Between 225 to 65 million years ago	Triassic	Between 225 to 190 million years ago
		Jurassic	Between 190 to 136 million years ago
		Cretaceous	Between 136 to 65 million years ago
CENOZOIC	Since 65 million years ago to the present	Tertiary	Between 65 to 2 million years ago
		Quaternary	Since 2 million years ago to today

 

MESOZOIC

Triassic to Jurassic

During the Mesozoic Era, the Western coast of Sonora was experimenting the subduction of an oceanic plate, equivalent to the diagram shown at right.

This subduction gave birth, between 215 to 180 million years ago (from the late Triassic to the Jurassic) to a line of volcanoes which followed an orientation North - South in our region. These volcanoes produced rhyolitic to dacitic lava spills, as well as numerous granitic and monzonite intrusives (that is, lava bodies which didn't erupt and cooled underground). Outcrops from this time period can be seen in the interactive map which appears further down in this text. To be able to see this map, you must have installed the Google Earth Plugin, which can be downloaded here.

In Nogales, Sonora, all along the Colonia Granja as well as towards the Cananea road, we have a granite outcrop (an orange colored rock, formed by grains of different tonalities), which correspond to this age, 150 mya (red delimited areas in the map)

North of Nogales, Arizona, Benedict Mtn was also formed by an intrusive (melted rock which didn´t come out to the surface as lava) corresponding to this age which, obviously, is the result of a magma body which didn´t came to the surface (green delimited areas in the map).

Jurassic to Cretaceous

Later, during the last stages of the Jurassic and beginning of Cretaceous, that is some 144 mya, vulcanism diminished, and our region turned into a zone of nonmarine deposits.

To see the following map, you must have installed the Google Earth plugin. You can get it here

 

Later, there would be a short time period when the sea invaded from the East to Sonora, turning the region around Nogales into the seashore of this marine basin. This shallow sea covered today´s States of Coahuila, Chihuahua and Eastern Sonora. From this time period are dated the marine fossils that can be found both, North of Nogales near Patagonia, Arizona, as well as South, east of Magdalena, near Cucurpe, in Sonora.

Cenozoic

Tertiary

Later, after the sea retreated towards the East, between 90 and 80 mya (late Cretaceous), it was succeeded by another Oceanic - Continental collision which again brought vulcanism with it. This vulcanism continued until some 52 mya, that is during the Tertiary. This volcanic activity is named Laramide Orogeny, and it´s process is very important economically, as it originated the mineral deposits, like Copper, which Northern Sonora and Southern Arizona have. This vulcanism was mainly andesitic (rocks of an intermediate color which came to the surface in explosive eruptions).

During this time period of intense volcanic activity, the first phase would give birth to the Sierra Madre Occdental, which is a layer of andesitic flows with a thickness of about one kilometer, forming a great plateau in central Mexico, and which extends North to cover also Chihuahua.

It would be followed by a period of geologic inactivity, until some 35 mya, when vulcanism reappeared. This last stage is named the Mid Tertiary Orogeny. It was divided in two phases: the first one, when rhyolitic rocks were expelled as lava (light rocks, which came out in an explosive form); followed by another kind of vulcanism, of basaltic rocks (black colored, which flow in a nonexplosive manner).

From the first stage, we have the hills located both East and West of downtown Nogales; and from the second stage, corresponding to basaltic vulcanism, we have in Nogales flows located both East as well as Southeast of the city: near Panteon de los Heroes, near Las Cuevitas (these rocks have been dated at 13 mya), near the water pumping station located towards Mascareñas, and La Pirinola hill.

By the way, La Prinola is not a volcano, but a basaltic flow which covered all that region, later was eroded, and left a small layer of basaltic rocks in top of that hill. At the right we can see La Pirinola, with a dotted line indicating where the floor was located at the time of this basaltic flow, some 13 million years ago.

As one gets close to La Pirinola, the layer of basaltic rocks will become apparent. From this time, there are still a few remaining basaltic volcanic craters that can be seen near La Arizona, as well as nearby the remains of basaltic flows will become visible.

This basaltic vulcanism was originated at the time of the end of the compression forces which had been exerted towards Western Sonora (when the the whole oceanic plate which had been colliding against it was subducted), and was replaced by a new set of tension forces, which stretched Sonora East-West, and also gave birth to the California Gulf.

This new process of stretching of Sonora gave birth to two new processes:

In Northern Sonora, today´s relief was born, which is formed by a set of parallel valley and mountain ranges, as is shown in the Municipality Topography webpage, and is shown in the following interactive map, in which are shown the faults separating our valleys and mountains.

In this map, the tooths appear on the fallen side of the block, while the dotted lines are inferred faults. We must take into account that these faults is only an approximation, inherent to the scale of this map.

In this interactive map we can see that the ranges in the Municipality, the same as in the rest of Sonora, also follow a Northeast-Southwest direction, and are separated by valleys. These ranges are bounded by normal faults, which also limit the intermediate valleys.

At the time of the formation of these mountain and valleys, the fallen blocks were transformed into closed watersheds, which gradually began to be filled by sediments from the nearby mountains. These sediments hardened gradually, until they formed a conglomerate.

It has been calculated that the sedimentation rate of this rock was of about 200 meters every million years. At the right can be seen an outcrop of this kind of rock, located in Southern Nogales, Sonora, along the top of Colonia San Carlos.

Within the municipality, this kind of rocks are known as Nogales Formation (dated from the second half of Miocene, between 5 and 15 mya), In particular, one outcrop of this kind of rock located near the border is known as "Cantera."

After these closed watershed became filled up with sediments, they began to communicate and drain towards the sea. This was the process that gave birth to the Santa Cruz River as well as its tributaries, one of them being the Arroyo de Los Nogales.

In the panoramic photo to our left, we can see the Santa Cruz Valley as seen from the heights of the Pinito Mountains. Left of the image are located the Santa Rita Mountains, and toward the right can be seen the San Antonio Mountains. The Santa Cruz Valley runs South to North (right to left in the image).

 

Outcrops within the Municipality

As has been shown in the previous text, in the Municipality can be seen outcrops of rocks from the following time periods. You can see the corresponding maps by clicking on their names, here:

Precambrian

Mesozoic

Tertiary

Quaternary

If you cannot understand some of the geologic concepts discussed here, you can read the following links:

• Plate Tectonics
• Geologic History of Sonora