Geologists’ Association field trip to Navarra and Aragon, N Spain
I worked in the foothills of the Pyrenees, N Spain for my PhD 1984-1987. I was excited to return there with The Geologists’ Association, September 2024.
Here is an introductory slide pack on the geology and landscape of Navarra and Aragon plus rare maps of the Jaca Basin (Cai Puigdefabregas 1973), Navarra Province (Diputacion de Navarra 1978) and the Pyrenees (Elf Aquitaine 1973).
View eastward from Gallipienzo (Antiguo), Navarra. High Pyrenees are visible in the distance. The hills in the foreground S of the Rio Aragon form the outermost topographic barrier separating thrust belt to the N from the undeformed Ebro basin. They are underlain by a major thrust complex that I worked on in the 1980s
Jaca basin topography in blue overlain with high-resolution digital elevation of four areas of interest visited during the GA field trip. 30x30m data from NASA; 2x2m DEMs from the Organismo Autonomo Centro Nacional de Informacion Geografica
Photo: G Churchard
Salinas de Oro
This is one of several Triassic salt diapirs along the Pamplona Fault, a major NE-SW oriented transcurrent lineament that marks the western limit of the Pyrenean foothills. In the core of the diapir are hundreds metres-sized outcrops of various ultramafics (peridotite) shown in blue on the geologic map. This combination of such disparate rock types is best explained as a record of the opening of the Bay of Biscay before the Iberian microplate collided with Europe to form the Pyrenees. New oceanic crust started forming in the Bay of Biscay since latest Jurassic times but it was preceded by a continental rift system that can be traced back to the Triassic. Extreme stretching and thinning of the continental crust would have downfaulted Triassic rift sediments (including salt) in the hanging walls of normal faults with mantle rocks in the footwalls (see seismic profile below from the analagous Iberia margin). Similar mantle rocks are exposed in the Arzacq and Mauleon basins to the N and along the North Pyrenean Fault, the original suture line between Iberia and Europe.
Map from the Diputacion de Navarra 1978; 2x2m DEMs from the Organismo Autonomo Centro Nacional de Informacion Geografica.
Ultramafic rocks exposed in the core of the Salinas de Oro salt diapir, Navarra. Photo: G Churchard
Seismic profile across the Iberia margin, Bay of Biscay. The data show low-angle normal faults downfaulting rift sediments in their hanging walls (downthrown sides) with lower crustal and mantle (green) rocks in their footwalls. From Pinto et al. 2023 - free to download from https://www.mdpi.com/2076-3263/13/12/374
Foz de Lumbier
A popular tourist spot in the area comprising sheer limestone cliffs, abundant griffon and egyptian vultures and a Roman villa just to the S of the gorge (“foz”). Geologically however there may be more to this feature than initially meets the eye. The gorge is situated where the Rio Irati cuts through an anticlinal fold in thick-bedded Paleocene limestone. But why didn’t the river keep to the more easily eroded soft Eocene marls (pale green on the map), a diversion around the W end of the anticline of just a kilometre? The answer is that the fold grew as the river cut down - see the ancient river scours in the photos, now uplifted high above the base of the gorge. The suggestion is that the anticline is a much younger structure that post-dates all the tectonic features in the area which owe their origin to the formation of the Pyrenees. There are several other features here that hint at young deformation: the anticline is oriented differently to the ‘Pyrenean’ trend (i.e. it’s more E-W than ENE-WSW), the area is traversed by prominent fault scarps typical of Recent tectonics (see the DEM below), and the southern margin of the anticline is bevelled by a S-dipping normal fault. How young is “young”? As always it’s difficult to date deformational structures but I suggest the fold and gorge formed in the Pliocene in response to late-stage adjustments of the thrust wedge.
Map from Cai Puigdefabregas 1973; 2x2m DEMs from the Organismo Autonomo Centro Nacional de Informacion Geografica.
Old meander bends now elevated high above the present river level, Foz de Lumbier, Navarra
Massive cliffs of Paleocene limestone stuffed with nummulites, Foz de Lumbier, Navarra
Reverse fault, Foz de Lumbier, Navarra
Narrow opening at the southern end of the Foz de Lumbier, Navarra
Petilla de Aragon
The birthplace of the Nobel prize-winning pathologist, neuroanatomist and pioneering microscopist Santiago Ramon y Cajal, Petilla de Aragon is indeed the petal of Aragon (despite its location within a small ‘island’ of Navarra in Aragon). Immediately S of the village is the Pena Flexure, a range of hills that mark the southernmost topographic barrier of the western Pyrenees foothills. The Pena Flexure is the along-strike continuation of the Sierras Exteriories, a structurally complex mountain front that formed where the regional thrust sheet ramped up and broke the ancient depositional surface. The geologic map shows the pinks, browns and greens of the westernmost tip of the Sierras Exteriores, where Mesozoic and Palaeogene rocks are thrusted up such that they crop out at the present surface. But what happens further to the W? All that shortening deformation accommodated by the thrust faulting exposed in the Sierras Exteriores can’t suddenly just end. The answer is a type of structure called a passive roof thrust. They typically form at the outer edges of thrust belts where the waning thrust system is running out of energy but at the same time is trying to widen the thrust wedge and push its tip further out into the foreland. Something eventually has to give. Nature’s solution is to form a passive backthrust (i.e. a thrust carrying its hanging wall toward the mountain belt) that develops in response to the underthrusting of the footwall (rather than more usual overthrusting of the hanging wall). I think Cai Puigdefabregas who produced the geologic map shown here understood the need to preserve shortening along strike from the Sierras Exteriores but didn’t know the solution. His 1973 map shows a strike-parallel lineament that could be the line of the passive backthrust although in all my years of fieldwork in this area I never found it - perhaps not surprising if the backthrust is parallel to bedding. However with the benefit of modern digital elevation data the trace of the backthrust can now be identified from surface lineaments. Passive backthrust structures have also been identified from the West Canada basin (seismic profile below), Kirthar and Sulaiman Ranges (Hindu Kush), and Peru where they are variously called intercutaneous wedges, triangle zones and even Pete Jones structures.
Map from Cai Puigdefabregas 1973; 2x2m DEMs from the Organismo Autonomo Centro Nacional de Informacion Geografica.
View S toward the 'Pena Flexure', a major topographic barrier separating thrust belt to the N from the undeformed Ebro basin on the other side of the flexure. The wall-like feature in the foreground is a vertically tilted sheet of fluvial sandstone. Petilla de Aragon, Navarra. Photo: G Churchard
Cross-sections through the Pena Flexure. From Turner JP 1990 Journal of the Geological Society 147 p177-184
Seismic profile through a passive backthrust structure, Canada
Arguis
N-S growth folds within the Sierras Exteriores, the range of hills that define the line along which the regional thrust sheet ramped up and broke the ancient depositional surface in the late Oligocene-Miocene. The term growth fold refers to structures which are active during sedimentation. In such settings the crests of anticlines form the high ground where sediment accumulation is restricted whilst the syncline cores would have been topographic lowpoints where sediments are thicker. The pale green and blue units in the geologic map comprise Eocene marine deposits that attained considerably greater thickness within the syncline cores. Note how the shallow marine facies (in blue) are mostly restricted to the mini-basin to the E of the nearer growth fold whilst their lateral equivalent in the next mini-basin to the W comprises deeper water (pale green) facies. How to explain N-S folds in a E-W thrust belt? I think they formed in response to rotation of the thrust belt due to the westward decline in the low-friction properties of the basal Triassic salt detachment. There are several lines of evidence in the region that the salt begins to ‘run out’ to the W meaning the thrust system would have got ‘stuck’, thus imposing a clockwise torsional strain hence a component of E-W shortening.
Map from Cai Puigdefabregas 1973; 2x2m DEMs from the Organismo Autonomo Centro Nacional de Informacion Geografica.
View westward toward the crest of the Arguis anticline. The saddle between the hills in the middle ground is the point at which Eocene marine sediments are at their most condensed squeezed between Paleocene limestone to the left and Oligocene sandstones - see geologic map and 3D perspective view. Mirador de Belsue, N of Arguis, Huesca. Photo: G Churchard
A few slides from the 1980s…
Los Mallos de Riglos, Zaragoza, Aragon
Sierras Exteriores from Gallipienzo
34 Million year old duck footprints, Javier, Navarra
Pena, pueblo abandanado
Normal faults don't get much clearer
My 'Dos Caballos', the perfect go anywhere vehicle for geologic fieldwork