Maquinaria requerida

Mingrano (758.2, 367.8 Lambert) is a small settlement found a short distance west o f the main road from Lorca to Caravaca de la Cmz, and the turning is located about 3 km NW o f La Paca (Figure 4.1 ).

The hills directly NW o f Mingrano (Figure 4.15) comprise altemating pale grey marly limestone and veiy pale grey/white marl beds o f the Pinosa Formation. The limestone beds frequently crop out and are clearly visible over the entire hillside. The beds are typically 30 to 50 cm thick and show no intemal stmcture or visible bioturbation. The marly beds are typically a few centimetres thicker and are exposed infrequently below limestone overhangs. The transition from marl to limestone is

C h ap ter 4 - Subbetic Deformation

gradual, with no sharply defined contact, and no discernible change in grain size. Microfossils are visible in the marls with hand lens.

The rocks are folded into a number o f anticlines and synclines trending NW-SE, and on different scales. Beds sampled belonged to the NE-dipping limb o f a syncline, with a range trend in dips from 1 5 °^0 3 0 to 30°->040, and a wavelength of several hundred metres. The lowest dip was collected from the hilltop. Near the SW foot o f the hill a temporary change in dip to 10°-^225 over a few metres defines a parasitic fold parallel to the main fold trend.

While folds in an obliquely convergent setting are not expected to form perpendicular to the shortening direction, the NW-SE trend o f these folds found NW of Mingrano is not expected in a broadly WNW-ESE to NW-SE compressional setting. The location is only 2-4 km SE o f the CFZ and is likely to have suffered rotation associated with middle-late Miocene slip on the fault, implying approximately 65°-90° o f vertical axis rotation since the folds were created.

The Pinosa Formation is dated by IGME as ranging from Middle Eocene to lower-most Miocene time, predominantly on samples collected further west (due to preservation issues). The Sierra de La Pinosa, located about 9 km west of the Mingrano outcrops, comprises very similar facies and a fauna that indicate an Eocene-Priabonian age in the oldest part. West o f the sierra the facies becomes marl dominated, and significantly easier to date. The biostratigraphers for the IGME memoir 931 were apparently in no doubt that the fauna supported the Middle Eocene to lowermost Miocene age range (Baena Pérez, 1974) in these marls. The memoir did report difficulties in dating this facies in some western areas, however, and reports that in places the fauna are entirely re-worked Cretaceous. Samples from this formation (VB4- VB12) to the west of Sierra de La Pinosa, in the Rambla del Cantar (746, 365 Lambert), were o f this latter type and yielded nothing but poorly preserved re-worked Cretaceous foraminifera. The outcrops in this area are very different in appearance to those found NW o f Mingrano and from the Sierra de La Pinosa, being very white, very fine-grained and much harder.

Samples ZR5, ZR6 and ZR7 (Figure 4.16) were collected from the hilltop (7523.5, 3692.0 UTM), as close as possible to the core o f the syncline in an effort to sample the youngest rocks. The best marl exposure is located in a 50 cm-thick bed on the hilltop directly beneath a limestone bed. Samples ZR8 and ZR9 were collected from the base o f the hill, SW o f the hilltop samples, and from stratigraphically lower beds.

Sample ZRl was collected from a location midway between the other sampling sites. Samples ZR6 and ZR9 were from the limestone beds and were thin-sectioned due to their competence. They proved o f little use in age determination.

The marl samples were rich in foraminifera but showed a significant amount o f CaCOs over-growth, making identification of foraminifera problematic, but a number of diagnostic species could be identified. Sample ZR5 yielded G lo b o q u a d rin a praedehiscens, Gq. baroemoenensis, Catapsydrax unicavus and G lo b ig erin a praebulloides. These are consistent with biozones N5-N6 o f Burdigalian time. Sample ZR8 yielded Globigerinoides triloba, Globoquadrina baroemoenensis, Catapsydrax parvulus and Globigerina druryi. These species do not tie the sediments down to a very

precise age, and merely suggest deposition during biozones N7-N15 from uppermost Burdigalian to lowermost Tortonian time.

The interesting thing about these results is the fact that while ZR8 is clearly younger than ZR5, the field relationships suggest the reverse should be true. A number o f possibilities exist to explain this, including human error in numbering the samples at the outcrop or in the laboratory during processing. Other alternatives require structural solutions, such as the presence of an extensional fault that has dropped ZR8 relative to ZR5, or the inversion of the stratigraphy during deformation.

The key thing that we gain from the samples in any case is that the pre­ deformation sediments NW o f Mingrano are clearly younger than previously thought. Sample ZR8 is likely to belong to pre-N9, and probably pre-N8 time due to the absence o f key Globorotalia species or other genera such as Orbulina or Praeorbulina that are common elsewhere in the Extemal Zones. While this assumption is speculative and based on absence rather than presence o f evidence it suggests that the main phase of compressive deformation in the area took place after some point in biozone N7 in uppermost Burdigalian time.

It is possible that samples from the Pinosa Formation should be divided based on the lateral variations, with the Rambla del Cantar-style reworked sediments separated and treated as of unknown age, because o f the absence o f outcrop showing clear relationships between the two styles o f sediment. With this in mind, automatic correlation between sites o f the same mapped formation either side o f Rambla del Cantar should be avoided and taken only on firm biostratigraphic age correlation.

C hap ter 4 - Subbetic Deformation