Publication: Fan-surface dynamics and biogenic calcrete development:
Interactions during ultimate phases of fan evolution in the
semiarid SE Spain (Murcia)
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1998
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Elsevier Science B.V., Amsterdam
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Abstract
Pleistocene alluvial fan surfaces of the Campo de Cartagena–Mar Menor Basin, Murcia, SE Spain. are capped by thick
mature calcretes. Calcrete profiles consist mainly of six different horizons: prismatic, chalky, nodular, massive, laminar and
coated-gravels. Petrographic study of the calcretes has shown the occurrence of features such as alveolar septal structures,
calcified filaments, coated grains, spherulites, calcified root cells and calcispheres that indicate the biogenic origin of the
calcretes, mainly induced by plant root related microbial activity. The calcretes studied were formed initially in the soil and
represented the K horizon. Development of the calcrete profiles took place in six main stages and was driven by multiple
phases of soil formation, erosion and reworking. The relationships between these processes caused the formation of different
calcrete profiles in proximal and distal fan areas. In the distal areas, which are controlled by limited distal fan aggradation,
episodic sediment input, buried previously developed calcretes and generated new space for calcrete growth by plants
growing in the overlying unconsolidated materials. This allowed the renewal of calcrete formation and it led to the
development of complex composite profiles which are thicker than in proximal areas, where surface stabilisation andror
dissection enabled calcrete reworking and brecciation. These processes of erosion, sedimentation, reworking and renewed
calcrete formation initiated by vegetation were repeated through time. They explain the complex macro- and microstructures
of these calcretes and indicate that calcrete development, even reaching mature stages, can start before the fan surface is
completely abandoned, but it requires episodic sedimentation. Eventually, distal fan aggradation and continuous calcrete
development throughout the entire fan surface, led to the ultimate fan surface induration, controlling subsequent landscape
evolution. So, fan surface calcretes cannot be envisaged as simple top-surface carbonate accumulations, but as complex
feedback systems in which pedogenic, biogenic and sedimentary processes interact in response to the evolving fan-surface
dynamics during the terminal phases of fan development in semiarid environments. q1998 Elsevier Science B.V. All rights
reserved.