Alluvial systems are sensitive sedimentary systems that can preserve useful information about tectonics, drainage area (lithology and size), and climate. However, research on this subject over the last three decades has made it evident that the interaction of these three factors and their control over the alluvial architecture and facies are complex. The stratigraphic and sedimentological analysis of the earliest syn-rift sedimentary record (Tithonian) in the West Cameros Basin has confirmed that sedimentation during the beginning of extension in this area took place in a series of halfgraben basins in which three different types of fan-shaped alluvial systems were deposited: two relatively large systems, specifically, a “highly channelized alluvial fan system” and a “distributive fluvial fan system changing downstream to an axial tributary fluvial system,” were the exception; and small “poorly channelized alluvial fan systems” were the norm. A semiarid climate is inferred during this earliest syn-rift sedimentary record from the steady and extensive calcrete development. This semiarid climate should play an important role both in how coarser sediment discharges to the rift halfgraben basins occurred and in the dominant transverse organization of the alluvial depositional systems. However, the climate and source-area lithology can be considered steady factors, so the main factors considered as the cause of these sedimentary and architectural heterogeneities are the tectonics and drainage-area sizes, and specifically, the different behavior of each extensional structure in terms of the rate of tectonic uplift and subsidence. This indicates how fault activity and drainage-area sizes during the earliest stages of a rift system can generate high degrees of heterogeneity in the architecture of alluvial basins and how this heterogeneity is recorded by the alluvial systems in the form of differences in their facies association and architecture. Therefore, this work concludes that both tectonics and catchment sizes exerted primary control on the sedimentary characteristics of associated alluvial systems in basins generated from this type of extensional setting, and care must be taken when making climatic and tectonic interpretations using the sedimentary record of these alluvial systems because many changes that traditionally have been attributed to climate variability can be explained solely by the differential behavior of each extensional structure and/or changes in its associated drainage area.