文摘
Small-scale siliceous sponge-microbialite buildups are recorded in the Middle–Upper Toarcian hemipelagic succession (marl–limestone alternations) of the Lusitanian Basin. A detailed study of these buildups from different outcrops recorded in the eastern area of the basin (Zambujal, Ordem, and Ro?a Cú) made it possible to characterize the typology of these buildups, identify the different components and facies, and discuss the controlling factors involved. The studied buildups developed in three growth stages: (I) a colonization stage of the muddy bottom by epibenthic macroinvertebrates, resulting in bioclastic patches providing a hard substrate, functioning as benthic islands; (II) a growth stage dominated by dish-shaped siliceous sponges encrusted by thin microbial crusts; (III) a growth stage dominated by thick microbial growth (thrombolites and minority leiolites) on siliceous sponges. This sequence is repeated in more complex buildups with cycles of growth, interruption, and resumption. The competition for available hard substrates among sessile benthic organisms, including microbial communities, reinforced the buildup growth with respect to the surrounding unfavorable soft substrate and controlled the high variability of the accompanying fauna. The installation of these buildups in the outer homoclinal ramp was favored by a low sedimentation rate, water depth, and type and availability of nutrients. Changes in the composition of sponge assemblages indicate decreasing dissolved organic carbon and colloidal substances along with increasing particulate organic matter, probably related with a shallowing-upward trend and a regressive phase.