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Source areas in the Agost Basin (Betic Cordillera)

07/26/2019 / Leave a comment

A new work to illustrate a changes in source areas related with pull-apart basin  in the Betics. Here the link to the work in researchgate.

Sedimentary and mineralogical analyses were performed in the Neogene Agost Basin (External Domain, Betic Cordillera) to reconstruct relationships between tectonics and sedimentation, and source areas evolution over time.

Geological Setting

Figure 1) A: Index map; B: Geological sketch showing the main zones and units of the Betic Cordillera; C: Geological map based on the main sedimentary cycles proposed by Vera (2004).

 

The sedimentary analysis allowed defining two sedimentary sequences: (1) Lower Stratigraphic Unit, Serravallian p.p. and (2) Upper Stratigraphic Unit, post Lower Tortonian (Upper Miocene p.p.)separated by an angular unconformity. They consist of marine (lithofacies L-1to L-3) and continental (lithofacies L-5to L-8) deposits respectively (Figure 2).

Figure 2. Lithostratigraphic record and correlations of four representative successions (logs 1 to 4) of the Agost Basin. The two main sequences (Lower Stratigraphic Unit, LSU and Upper Stratigraphic Unit, USU) separated by an angular unconformity are evidenced. Depositional environments, supplies and the isochronous lines T1-T5 are also indicated.

 

The analysis of mineralogical assemblages and some XRD parameters of the sedimentary sequences (Figure 3) and older formations (Figure 4) allowed recognizing a sedimentary evolution controlled by the activation of different source areas over time.

Figure 3. Mineralogical results of the Neogene sedimentary record of the Agost Basin (LSU and USU, given as the average value from the set of samples (n) in each lithofacies L-1 to L-8 included in tab. 2 for the whole rock and the <2 µm grain-size fraction (in wt. %). Ranges and average values of the intensities ratio of the Qtz(001)/Qtz(101) peak areas of quartz, and Sme(003)/Sme(002) peak areas of smectite and Ill(002)/Ill(001) peak areas of illite under ethylene glycol solvation are included.

 

Figure 4. Mineralogical results concerning the source areas given as the average value from the set of samples (n) in each Sedimentary Cycle included in tab. 1 for the whole rock and the <2 µm grain-size fraction (in wt. %). Ranges and average values of the intensities ratio of the Qtz(001)/Qtz(101) peak areas of quartz, and Sme(003)/Sme(002) peak areas of smectite and Ill(002)/Ill(001) peak areas of illite under ethylene glycol solvation are included.

 

In particular, the Ill+Kln±Sme+Chl clay-mineral association characterizes the supply from Triassic formations; the Ill+Kln+Sme association from Albian formations; the Sme+Ill±Kln+(I-S) and Sme+Ill±Kln associations from Upper Cretaceous p.p.formations; and the Sme+Ill±Kln+(I-S) association from Paleogene formations, testifying a tectonic mobility of the basin margins differentiated over time (Figure 5).

 

Figure 5. (A) Comparative plots of XRD parameters, traceable mineral phases, and clay-mineral associations of the Neogene sedimentary record of the Agost Basin (LSU and USU, including the lithofacies L-1 to L-8) and source areas (Sedimentary Cycles I to VI as Triassic p.p., CI; Albian p.p., CIV; Cenomanian-Turonian p.p., CV-CT; Upper Cretaceous p.p., CV-S; and Paleogene p.p., CVI); purple, green, and orange dotted lines identify supplies from predominant Triassic, Cretaceous, and Upper Cretaceous-Paleogene source areas, respectively. The ternary plots for the whole-rock (B) and the <2 µm grain-size fraction (C) mineralogy, given as the average value (in wt. %) from the set of samples in each Sedimentary Cycle (tab. 1, fig. 4) and lithofacies (tab. 2, fig. 5), show three compositional fields corresponding to predominant Triassic like (TR), Albian like (AL), and Upper Cretaceous-Paleogene like (CP) mineralogical associations mixing in variable proportions to determine the mineralogical associations from L-1 to L-8. Samples with gypsum and chloride as typical Triassic mineral phases to identify the Triassic influence in F-1 to F-8 are indicated.

 

This reconstruction leads to propose detailed relationships between types of deposits and provenance and not a classic “unroofing”, as follows: (i) the lithofacies L-1 (lithofacies L-2 and L-3 were not analysed) is characterized by the Ill+Kln+Sme mineralogical association indicating an origin from the Albian formations; (ii) the lithofacies L-4 shows a mixture of Ill+Kln+Sme and Sme+Ill+Kln associations sourced from the Albian and Upper Cretaceous formations; (iii) the lithofacies L-5 is characterized by the Sme+Ill±Kln+(I-S) association indicating a provenance from the Upper Cretaceous and Paleogene formations; (iv) the lithofacies L-6 to L-8 are characterized by the Ill+Kln±Sme+Chl association indicating a supply mainly from Triassic deposits. The evolutionary sedimentary model reconstructed for the Agost Basin, which improves a previous contribution about the same area, has been correlated with those reported in other intramontane Neogene basins in the Betic-Rifian Arc studied with similar resolution, so obtaining useful information for regional reconstructions.

Figure 6. Tectono-sedimentary evolutionary model with location of source areas of the Agost Basin and surrounding areas. A: Middle Miocene stage (Lower Stratigraphic Unit); B: Late Miocene stage (Upper Stratigraphic Unit).

 

 

Cite as: Martín-Martín, M., Guerrera, F., Alcalá, F. J., Serrano, F., Tramontana, M. (2018): Source areas evolution in the Neogene Agost Basin (Betic Cordillera): implications for regional reconstructions. Italian Journal of Geoscience. 2018, 137(3): 433-451. doi:10.3301/IJG.2018.14

 

The Agost Basin (Betic Cordillera, Alicante province, Spain): a pull-apart basin involving salt tectonics

07/10/2019 / Leave a comment

A new work to illustrate a strike-slip basin in the Betics. Here the link to the publisher.

The Agost Basin is characterized by a Miocene-Quaternary shallow marine and continental infilling controlled by the evolution of several curvilinear faults involving salt tectonics derived from Triassic rocks. From the Serravallian on, the area experienced a horizontal maximum compression with a rotation of the maximum stress axis from E-W to N-S.

Figure 1) A: Index map; B: Geological sketch showing the main zones and units of the Betic Cordillera; C: Geological map based on the main sedimentary cycles proposed by Vera (2004).

 

Geological setting

Figure 2) A: Detailed geological map of a sector of the Novelda-Jijona Strike-Slip Fault zone including the Agost Basin and surrounding areas (location shown in Fig. 1C); B: Detailed geological map of the Agost Basin, including the five isochronous lines T1-T5.
Figure 3) Geological cross sections (located in Figure 2) showing the lateral structural evolution of the Novelda-Jijona Strike-Slip Fault. The insertion of isochronous lines T1-T5 is useful for lateral correlations in the Agost Basin.

The resulting deformation gave rise to a strike-slip fault whose evolution is characterized progressively by three stages (see Figure 4): (i) stepover/releasing bend with a dextral motion of blocks; (ii) very close to pure horizontal compression; and (iii) restraining bend with a sinistral movement of blocks.

Figure 4) Paleogeographic model of the Agost Basin and surrounding relief with indications of sedimentary environments and isochronous lines T1-T5.

In particular, after an incipient fracturing stage, faults generated a pull-apart basin with terraced sidewall fault and graben subzones developed in the context of a dextral stepover during the lower part of late Miocene p.p. The occurrence of Triassic shales and evaporites played a fundamental role in the tectonic evolution of the study area. The salty material flowed along faults during this stage generating salt walls in root zones and salt push-up structures at the surface. During the purely compressive stage (middle part of late Miocene p.p.) the salt walls were squeezed to form extrusive mushroom-like structures. The large amount of clayish and salty material that surfaced was rapidly eroded and deposited into the basin, generating prograding fan clinoforms. The occurrence of shales and evaporites (both in the margins of the basin and in the proper infilling) favored folding of basin deposits, faulting, and the formation of rising blocks. Later, in the last stage (upper part of late Miocene p.p.), the area was affected by sinistralrestraining conditions and faults must have bent to their current shape. The progressive folding of the basin and deformation of margins changed the supply points and finally caused the end of deposition and the beginning of the current erosive systems. On the basis of the interdisciplinary results, the Agost Basin can be considered a key case of the interference between salt tectonics and the evolution of strike-slip fault zones. The reconstructed model has been compared with several scaled sandbox analogical models and with some natural pull-apart basins.

Figure 5) Comparison with scaled analogical models and natural pull-apart basins. A: sandbox analogic model, non-overlapping releasing sidestep, to simulate the kinematic and geometric evolution of pull-apart basins, after Dooley and McClay (1997); B: Sketch map of the Dead Sea Basin (from Keydar et al. 2013, modified); C: Sketch map of the Salina del Fraile in Argentina (from Reijs and McClay, 2003, modified); D: Sunik pull-apart basin in Armenia (from Karakhanian et al., 2002, modified).

Research supported by: Research Project CGL2016-75679-P, Spanish Ministry of Education and Science; Research Groups and Projects of the Generalitat Valenciana, Alicante University (CTMA-IGA); Research Group RNM 146, Junta de Andalucía; Grants from University of Urbino “Carlo Bo”, responsible M. Tramontana.

Cite as: Martín-Martín, M., Estévez, A., Martín-Rojas, I., Guerrera, F., Alcalá, F. J., Serrano, F., Tramontana, M. (2018): The Agost Basin (Betic Cordillera, Alicante province, Spain): a pull-apart basin involving salt tectonics. International Journal of Earth Sciences. 107, 2: 655-671. Doi: 10.1007/s00531-017-1521-6