Francisco Delgado

Triassic continental rifting of Pangaea: direct evidence from the Alpujarride carbonates, Betic Cordillera, SE Spain

Abstract: Evidence is presented for Triassic rift-related palaeo-structures from the Alpujarride Complex carbonates of the Betic Cordillera, SE Spain. Direct evidence of synsedimentary extensional tectonics is provided by macro- to meso-scale normal faults overstepped by younger strata. Most faults define domino-like horst and graben structures. The faults are associated with an expanded synrift sedimentary prism in which soft-sediment deformation, gravity-flow deposits and unconformities are widely developed. Syntectonic mafic igneous intrusions also occur. The age of this phase of extension is constrained as Ladinian–Carnian (c. 237–216 Ma). From a palaeogeographical standpoint, the thick Triassic carbonates of the Betic Internal Zone, together with comparable successions in Northern Calabria (Southern Apennines), occupied a belt with elevated subsidence connecting the Neo-Tethys to the east with the eastern North American intracontinental rift system to the west. Their carbonate facies, intermediate between classical Alpine- and Germanic-type Triassic facies, recorded the main episodes of rifting affecting Central Pangaea.

Biostromes of dasycladacean algae and stromatolites: A peculiar interbedding

Abstract An uncommon interbedding: Dasycladacean algae—stromatolites appears to be represented in dolomitic materials, Norian in age, of the Trevenque nappe (Alpujarride Complex, Betic Belts). Dasycladacean facies consist of massive (several metres thick) flattened masses, parallel to bedding, the lateral extent of which is commonly in excess of 100 m. A close view reveals Dasycladacean alga stems to be oriented perpendicular to bedding and interconnected with organic encrustations and cements of presumed marine origin. Stromatolite facies also have a stratiform disposition and their dimensions are of the same range as that of dasyclads. Their morphology is of the LLH (Lateral Linked Hemispheroids) variety of Logan et al. (1964). They appear to have been lithified early in a shallow marine environment. Stromatolite breccias are locally abundant. The contact between these two major types of facies is usually sharp, clear and plane. They seem to grade towards E-SE to calcarenite bar deposits, which probably caused restriction on the environment in which dasyclads and stromatolites developed. The dasyclads inhabited a shallow, protected and restricted lagoon, building biostromes on top of which stromatolites implanted themselves. Periodic subsidence of the bottom caused new cycles (Dasycladacean—stromatolites) to appear. Nine of these cycles can be recognized.

The Triassic platform of the Gador-Turon unit (Alpujarride complex, Betic Cordillera, southeast Spain): climate versus tectonic factors controlling platform architecture

A litho-biostratigraphic analysis has been carried out in the Gador-Turon unit of the Sierra de Gador (Alpujarride complex, Betic Cordillera, SE Spain). The Triassic succession of this unit is composed of a lower meta-detrital formation overlain by an upper meta-carbonate formation divided in six members. In the latter, a Ladinian–Carnian-rich fossil association has been found (foraminifers, algae, bivalves, microproblematica, trace fossils). Facies analysis has enabled the recognition of 22 facies of platform origin. This succession accumulated as a subsiding margin-type carbonate platform with homoclinal ramp geometry (Anisian?–Ladinian) evolving into a fault-block-type platform with a steeper-margined geometry (Ladinian–Carnian). Slope deposits of this latter platform show a prism-like geometry with progradational patterns and include syn-sedimentary structures associated with normal faults capped by younger beds. The results of the present research indicate that the architecture of the platform studied has been controlled mainly by climate and oceanic factors during the development of the ramp, and by syn-sedimentary extensional tectonics during the development of the steeper-margined platform. The Ladinian–Carnian tectonic activity was probably also responsible for the siliciclastic input and the shift to a mixed terrigenous-carbonate platform.