Sedimentary and tectonic evolution of the Douro coalfield basin (Lower Stephanian C, NW Portugal) [Evolu¿c¿ao sedimentar e tect¿onica da Bacia Carbo¿ifera do Douro (Estefaniano C inferior, NW de Portugal)]

• Type: Article in International Scientific Journal
• Date: 2003

Authors

A. Pinto de Jesus (Author) FCUP

Journal

Title: Cadernos do Laboratorio Xeoloxico de Laxe

Pages: 107-125

ISSN: 0213-4497

Publisher: Seminario de Estudos Galegos

Indexing

Scopus - 28 Citations

Other information

Authenticus ID: P-007-BP8

Abstract (EN): Facies analysis and tectonosedimentary analysis methods were used to study the Sedimentary record of the Douro Coalfield Basin (NW Portugal). The data obtained was integrated with alocyclical processes, mainly those resulting from tectonic and/or magmatic activity. For interpretation the regional geological setting was taken into consideration. Consequently, the main tectonic structures and data related to the magmatism for the region were taken into account. Furthermore, in order to account for the thermal evolution of the coals and other rocks of the Douro Coalfield Basin, organic and sedimentary petrographic elements as well as geochemical and mineralogical controls for the illite crystallinity based on the Kubler Index were taken into account. Integrating all the elements and data obtained we present one model for the genesis and evolution of the Douro Coalfield Basin. Thus, after the formation of the pull-apart basin, controlled by the sinistral shear activity of the Douro-Beira Carboniferous Trough after the upper Westphalian D, the depocentres migrated from NW to SE. Changes in tectonic settings allowed for the development of "foreland style" Sedimentary basin with input from alluvial fans, structured at the margins of the Douro Coalfield Basin, providing sediments into a trough with palustrine/lacustrine characteristics where important deposition of vegetation took place. These structures were strongly cut and highly reworked by multistory-multichannel trunk braided river downstreaming northwestwards. The tectonic activity at the NE margin lead to an asymmetrical basin deepening northeastwards. This process and the decrease in the fluvial hydrodynamic energy resulted in the formation of palustrine/lacustrine depositional systems, first on the floodplains, and then filling all of the Douro Basin. Sedimentary records suggest that sedimentation was syntectonic throughout the series, with a progressive decrease in energy levels. The implantation of granitoid rocks in the region and the resulting thermal contribution, concomitant with the downlift of the Douro Coalfield Basin, lead to the coalification of the coal measures allowing these to reach the meta-anthracite range of rank. At the same time, late variscan phases acting over brittle rocks (exception made to the coals), with a stress torque oriented NE-SW, and associated with sinistral shear activity, resulted in the formation of thrust faults with a "piggS-back" style and the subsequent deformation forming duplex and triplex tectonic structures. The obliqueness of the tectonic movement resulted in (diagonal) asymmetrical cut of the upper series of the Douro Coalfield Basin for the major overthrust fault. Also, related at this framework, at least in one mine (Germunde colliery), the rotation (plunging southeastwards) of the coal strata nearest to the major overthrust fault was recognized. Microfolds were observed in the coals, and other associated rocks showed evidence of cleavage and of brittleness processes. Syntectonic sedimentation, as manifested in the sedimentary record and in the structure of the Douro Coalfield Basin, strongly suggests that, in the Douro-Beira Carboniferous Trough, the regime changed from a transcurrent sinistral NW-SE one during Westphalian times to a transpressive sinistral regime with a W-E compressive component. The latter increased in intensity, becoming predominant over the sinistral shear component already during the lower Stephanian C and earlier times of the Variscan Orogeny.

Language: Spanish

Type (Professor's evaluation): Scientific