In October 2007, nine undergraduate and graduate students and five faculty from the University of Alabama traveled to La Popa basin, in northeastern Mexico, to examine superbly exposed structures and Lower Cretaceous to Lower Tertiary strata that record mobilization of Upper Jurassic Minas Viejas Formation evaporites. Diapiric evaporite structures in this basin are among the best-exposed analogs to subsurface hydrocarbon traps in the Gulf of Mexico and elsewhere. Objectives of this field-course were to:
- Obtain first-hand experience observing and interpreting the structural and stratigraphic features associated with evaporite diapirs
- Compare and contrast structure and stratigraphy associated with passive evaporite diapirism versus evaporite-detachment structures (evaporite-cored anticlines)
The University of Alabama Department of Geological Sciences is grateful
for generous financial assistance from George Lindahl, III
Students and faculty in The University of Alabama Fall 2007 Seminar in Tectonics [GEO 534/634] spent 4 days (Oct 11-14) examining stratigraphic and structural relationships northwest of Hidalgo, Mexico. Rental vans were used to obtain access to well-exposed geologic features in the mountainous desert along the east flank of the Sierra Madre Oriental. The participants spent three nights at Posada El Potrero Chico on the outskirts of Hidalgo.
DAY ONE: An overview of stratigraphy and structural features in the Coahuila Foldbelt and La Popa Basin, Hidalgo to La Popa
- We took a look from a distance at the basins, lentils, and folded strata
Pics and Descriptions:
Our first roadside stop in Mexico just outside of Hildalgo to get an overview of the days to come:
This caliche was found on the roadside near the picture taken above. Based on the carbonates in the area they must be younger than Cretaceous in age. Caliche, most often found in desert and semi-arid regions, is formed from two processes: meteoric water and the rise and fall of the ground water table. The picture shows the hardened caliche cementing the larger minerals:
Here is a view almost straight along axis of carbonate folds. This feature is a doubly plunging anticline with the top of the mountain being the culmination:
On our way back to base camp we stopped at an abandoned mission. Notice the carbonate lentils in the background:
Another view at the mission:
The objective of this field day is to observe the stratigraphic relationship between the El Papalote Diapir and the carbonate lentils (lenticular marine carbonate beds) adjacent to it. The lentils are depositionally thicker and more deformed near the diapir and are characterized by conglomerate with meta-igneous and limestone clasts. TheseÂ conglomerates are typically overlain by red algal wackestone to packstone or oyster wackestone.
Overview of Day 2
- Examined Lentil 1 adjacent to the El Papalote Diapir on the northern rim
- Hiked southeast to observe meta-volcanic and limestone blocks in the diapir, as well as allochthogenic blocks of the Zuloaga Formation
- Observed green and purple cataclasite with a gypsum matrix and massive gypsum with selenite veins in a shear zone in an arroyo in the diapir
- Hiked south to Lentils 2 and 3
- After leaving the El Papalote Diapir, hiked to a near-by outcrop of the Delgado Sandstone Tongue where possible tsunami deposits mark the K-T Boundary on the northeastern limb of the El Gordo anticline
After leaving our base camp at Potrero Chico in Hidalgo, Nuevo Leon, we drove north on Hwy. 53 to kilometer post 50 and turned left (west) on a dirt road. After the explosives sign, bear to the right at the Y intersection and park where the road ends which is very close to Lentil 1 and the northern rim of the diapir.
The El Papalote Diapir is an exposed Jurassic massive gypsum deposit which is located in the La Popa Basin in northeastern Mexico. La Popa Basin is a distal part of the Hidalgoan foreland system which includes the Sierra Madre Oriental fold and thrust belt.
El Papalote Diapir area.Â Lentil 1 outcrop.
Meta-igneous block exposed within the diapir.
Exposed K-T Boundary in outcrop near the El Papalote Diapir.
Panoraic shot overlooking the El Papalote Diapir.
Day 3: Evidence of halokinesis (salt mobilization): La Popa Weld
La Popa salt weld was visited near El Toro lentil at Boca La Carroza; San Jose lentil and the MuertoFormation
- Examined the salt evacuation and weld formation stage
- Visited the remnant salt wall at the northern end of the weld
- Observed structural and stratigraphic response to syndepositional salt evacuation
Departing from base camp at Portrero Chico, we visited the La Popa salt weld at two locations and visited the remnant La Popa salt weld at the northern end of the weld.
The first stop was at an arroyo where we were able to enjoy an up-close view of the disconformity resulting salt evacuation and weld formation.
The second stop was at the remaining salt (gypsum) on the north end of the weld in the village of San Jose de la Popa.
The third stop was at the base of the ridge capped by San Jose lentil where we were able to observe evidence of syn-depositional salt evacuation.
Cross section and lithologic descriptions from Shelley and Lawton (2005):
Kl : Lower Cretaceous Limestones : marine limestones; El Toro lentil, Boca de Carroza lentil
Ki : Indidura Formation : silty marine limestone; El Toro lentil
Kpa : Parras Shale : marine shale
Km : Muerto Formation : deltaic sandstone and base of the Difunta Group
Kpl : Lower Portrerillos Formation : includes the lower marine siltstone member of the Difunta Group; the lower marine mudstone member of the Difunta Group, which is associated with the Lower Gordo lentil and San Jose lentil; the Cuchilla sandstone tongue (marine sandstone); the middle marine siltstone member of the Difunta Group; and the Delgado sandstone tongue with tsunamite deposits as the uppermost member of the Lower Portrerillos Formation
Tpu : Upper Portrerillos Formation : includes at its base the upper marine mudstone member of the Difunta Group, which is associated with the Upper Gordo lentil, La Popa lentil, and North Chivos lentil; and a tide-dominated upper sandstone member (Tpss) as the top of the Upper Portrerillos Formation
[Tpss : the tide-domonated upper sandstone member at the top of the Upper Porterrillos Formation, displayed as a seperate unit because this was the topic of the Shelley (2005) article]
Ta : Adjuntas Formation : fluvial Difunta Group deposits overlying the Upper Portrerillos
Tv : Viento Formation : marine sandstone Difunta Group deposits overlying the Adjuntas Formation
Stop 1: La Popa Weld, Arroyo Outcrop
View up the arroyo, with outcrops on the left. This sequence of photos displays the transect we made along the outcrop:
Heading up the arroyo, we first encounter marine limestones of the El Toro lentil (packstones and wackestones) with abundant fossils:
Portion of the limestone rich in bivalve fragments:
Cutaway and plan views of internally molded spiral gastropod shells are preserved in the endurated sandstone:
This marine limestone unit also frequently contained centimeter-scale calcite fracture-fill, at times displaying beautiful en-echelon fabrics:
The marine limestone unit also contained clasts of rounded chert clasts which was observed in numerous cobbles from the arroyo, though not observed in outcrop. Ribbon cherts were observed in outcrop, interbedded with marine muds, shell lag deposits, and siliciclastics:
As we continued up the arroyo, the next lithology encountered was a fine sandstone with centimeter-scale fracture fill:
Followed by an strongly foliated mudstone:
We next encountered a conglomerate/debris flow deposits with terrigenous clasts of decimeter scale:
Overview Panorama of the La Popa Weld, Arroyo Outcrop (Click to enlarge):
Stop 2: La Popa Salt Wall Remnant
Gypsum exposed (at surface, forming poor soils) in the village of San Jose de la Popa:
Stop 3: San Jose lentil and the Muerto Formation
The San Jose Lentil caps the ridge below:
Sandstone beds of the Muerto Formation thin towards the weld, and are truncated, indicating halokinesis during deposition of the Muerto beds:
Overview Panoramas of San Jose lentil and the Muerto Formation
(Click to enlarge):
Last Day in Mexico
- Pack up and head into the basin for the morning
- A nice view of the vertically dipping limestone beds and a look at more diapiric evaporite
Pics and Descriptions:
Closer view of flat irons at entrance to Potrero Chico:
Inside the Potrero Chico anticline looking at the pentration of gypsum
into the carbonate beds shown previously:
Close view of gypsum inside Potrero Chico showing a deformed fabric
suggesting the evaporite is diapiric and not part of a detachment structure: