Benthic foraminiferal morphogroups and their response to relative sea-level changes in the upper Hauterivian-lower Barremian Agua de la Mula Member of the Agrio Formation (Patagonia, Argentina)

A morphogroup analysis of the upper Hauterivian-lower Barremian foraminiferal assemblages from the Agua de la Mula Member of the Agrio Formation (Neuquén Basin, Argentina) reveled six associations based on benthic foraminiferal morphogroups identified by means of cluster analysis and treated with multivariate methods. In order to assess the paleoenvironmental significance of the morphogroup associations, they were evaluated in relation to sampled geographic localities, ammonite biozones, sedimentary facies, and position into depositional sequences of the five sections analyzed. For this purpose, the statistical techniques of RDA, partial-RDA, PERMANOVA, and PCoA were applied. The morphogroup associations MgFA1, MgFA4, MgFA5 and MgFA6 revealed high percentages of planoconvex, biconvex and conical forms, with an inferred epifaunal habit and active herbivorous feeding strategy. The remaining MgFAs2 and 3 are mainly represented by planispiral tests with lenticular, flattened and elongated forms, attributed to shallow infaunal lifestyle and detrital and bacterial scavenger feeding habit. Sedimentation rates and associated food availability were considered the main controlling factors in the distribution of morphogroups across the analyzed tract of the basin, revealing differences between proximal and distal sectors. In the sections characterized by higher terrigenous input, in a most proximal position of the basin, food increases downward in the sediments and so shallow infaunal morphogroups prevail (MgFAs2–3). At the same time, shallow infaunal-dominated associations are distributed throughout 3rd-order regressive systems tracts that commonly are associated with high sedimentation rates. Associations mostly represented by epifaunal morphogroups (MgFAs1, 4–6) recur at 3rd-order transgressive systems tracts where low sedimentary input characterizes the sequences, favoring long exposure of organic matter at the seafloor. Moreover, epifaunal forms with grazing herbivorous and detritivorous habits increase throughout the distal sector of the basin, following the decreasing supply of organic matter in settings farther from sedimentary input.