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The geometry of a sector in the groundwater basin of the Tulum Valley has been studied to determine the shape, thickness, and vertical and horizontal distribution of the grain size, as well as the depositional environmental conditions of the Quaternary deposits that fill the valley. The geomorphologic features of the area have been investigated on the basis of aerial photographs checked with fieldwork. Three subsurface sections were prepared for a hydrogeological analysis of the area. These cross-sections were prepared by combining information from descriptions of well samples and interpretations of geophysical logs of wells and electric resistivity surveys. Within the studied area, the floor of the groundwater basin is asymmetrically shaped; the Quaternary deposits, which lie on an impervious or poorly pervious electrically conductive hydrogeologic basement of Late Tertiary age, reach a thickness of 670 m in the west and only 215 m in the eastern extreme. The Tulum Valley Basin is divided into two subbasins by a fault system trending NNE-SSW, which plays an important role in the configuration of the basin and the distribution of the Quaternary sediments units, as well as the distribution of aquifers in the subsurface. The western subbasin has a thicker cover and coarser grain sizes than the eastern one, where the sediments have more fine-grained intercalations and hardpans. The latter are probably pedogenic in origin. (c) 2006 Elsevier Ltd. All rights reserved.