Two days of mapping in the Baca Canyon, West rift margin

(May 31 to June 1)

by Gwenaël CARRER and Jérémie RUEST

Baca Canyon rains part or the northeast end of the Bear Mountains of Socorro Country, north of Hell’s Mesa. It is the west part of the Rio Grande rift and we can find witnesses of the activities of the rift.

Description of the formations

At the bottom of the stratigraphical organization, there is a fine to medium grained lithic sandstone with tough cross-stratification and lateral accetion surfaces; fuvial deposits. This formation named Crevasse Canyon formation is from Upper cretaceous and is a member of Meseverde group. The height is 300 m.

Then, there is reddish orange to reddish brown mudstone with interbedded red, yellow, gray, and white sandstone and conglomerate; fluvial lacustrine deposits. Some units resemble underlying Crevasse Canyon formation but Baca strata are coarser grained and overlie a thick redbed succession. This formation named Baca formation is from Eocene and its height is 0 to 900 meters.

Then, we can find an volvanic event. It is purple to grey volcanic conglomerate, volcaniclastic sandstone, and minor mafic of intermediate lava flows; characterized by outsized white phenocrysts and clasts of plagioclase feldspar; Alluvial, braided fluvial, and lacustrine. Deposits coarsen section and contain more lava flows upsection. This formation named Spears formation is from Eocene.

Finally, at the top of the stratigraphical organization, there is gray quartz-rich and cristal- rich latite to rhyolite ashflow-tuffs. There are multiple flows densely welded. This formation named Hell’s Mesa Tuff is 32 millions years old.

Sedimentary and volcanic rocks of late Cretaceous through early miocene age crop out in the study area and display effects of mid –late Tertiary faulting. Many faults have been intruded basalt to basaltic-andesite dikes.

Description of mapping area

We saw beautiful cross stratification in Crevasse Canyon formation (Figure 2) who indicate a polarity and the environmental deposit.

a) b)

Figure 2 : Detail of cross stratifications in Crevasse Canyon a) Photography b) Interpretative sketch

The witnesses of the tectonic were visibles. For example, there were striped quartz plans (Figure 3) and we found normal faults (Figure 4). The strike of the normal fault is North-South and the deep is to the West. So, we can correlate it with the activity of the rift.

a) b)

Figure 3 : a) Striped quartz plan and the b) interpretative sketch.

a) b)

Figure 4 : Normal fault in Baca formation. The dyke is cut as the interpretative sketch b)

But it is important to notice that in a West mountain there was a normal fault with two deep when we moove along the fault : West then East. It was located in Baca formation and Spears formation.

We saw coal level in Baca formation. It is a new information to say that we are in fluvial anoxic area of deposit.

We saw dykes with many feldspath and pyroxene and olivine. The prism appear but the loess appear less. One dyke had brecchia so we can think that the tectonic began before it.

Synthesis

The majority of the formation are fluvial deposit. The cross stratification show an alluvial delta. The cretaceous fluviatil deposite had been eroded during the paleocene and covered by Baca formation.

After Crevasse Canyon and Baca Canyon formation, there is a volcanic events who was recuperated to create fluvial sediment. It is Spears Formation. The volcanic event went on Hell’s Mesa tuff but with an other shape. The fault is 32 millions years old. The intrusion began since 25 millions years ago in this fault. The tectonic allowed the activity of the faults and worked when the intrusion came on.

This area carry the stigmat of the Rio Grande rift because of the tectonic.

Baca Canyon

R J, C G