The Cretaceous La Luna Formation at the Middle Magdalena Valley Basin is a classic shale-gas system in which the rock is the source, reservoir, and seal. It was deposited in a deep water marine environment with little oxygen in the sea floor. The Galembo Member comprises several lithofacies dominated by finegrained (clay- to- silt-size) particles. Five lithofacies are recognized based on mineralogy, fabric, fossil content and texture: (1) nonlaminated to slight laminated foraminifera wackestones, (2) moderate to well-laminated highly fossiliferous claystones rich in organic matter; (3) claystones with fossiliferous carbonate concretions with pyrite; (4) nonlaminated siliceous and fossiliferous claystones; (5) ash falls. Each facies contains abundant pyrite and phosphate. Carbonate concretions are also common. The fossil content shows a predominantly deep marine paleoenvironment of deposition. Minor changes can generate variations in the relative proportion of terrigenous material, precipitation of organic matter anddiagenetic alterations. Its deposition is estimated to have occurred over a 25- m.y. period, and despite the variations in sublithofacies, sedimentation style remained remarkably similar throughout this span of time.&nbsp