Those exploring for oil in the Galisteo Basin are concentrating their efforts on rocks laid down during the Cretaceous Period, a time when Northern New Mexico was covered by a vast inland sea that stretched from the Arctic to the Gulf of Mexico.

To the west were mountain ranges, to the east a low-lying land mass. Material was eroded from the land masses and formed deposits some 3,000 feet thick in what is now Northern and northeastern New Mexico.

The shorelines of this seaway fluctuated. "It's a classic example of the sea came in; the sea went out," said Steve Cather, a senior field geologist at the New Mexico Bureau of Geology and Mineral Resources in Socorro.

As the water became deeper or shallower, several different kinds of sediments were deposited, thus the different geologic formations, which are often named after the geographical area where they were first described by geologists.

About 95 million years ago, what is now the San Juan Basin was relatively close to shore, and the sands of the Dakota Formation were deposited. Some 85 million years ago, the sea was deeper; plankton fell to the bottom, becoming the hydrocarbon-rich Mancos Shale.

About 80 million years ago, the deeper waters receded, and the more terrestrial sediments of the Mesaverde Group were deposited. A seashore with its related sandy barrier beaches moved back and forth across the area, becoming the Point Lookout Sandstone. Later in Mesaverde time, the area was a swampy lowland, alternating between stagnant bays, winding rivers and beach strands, forming the coal deposits of the Menefee Formation.

The ages of the formations vary from one part of the San Juan Basin to the other. For instance, as the sea receded, it took about 5 million years for the coast to traverse the basin, laying down sandy shorelines.

Over the ages, the marine, seashore and swamp deposits were buried and lithified, or converted to rock.

The Dakota deposits became a sandstone that trapped oil deposited earlier in the Mesozoic Era — the age of dinosaurs. The plankton-rich Mancos Shale became a good source rock for oil and gas. The Point Lookout and other sands of the Mesaverde group became traps for hydrocarbons. Swamps and sandbars of the Menefee became sandstones, shales and coal beds — a source of coal-bed methane near Raton and Farmington.

About 75 million years ago, the Cretaceous sea withdrew to the north and south of our area. Tectonic forces caused compression in the earth's crust, and the Rocky Mountains began to rise, in what geologists call the Laramide Orogeny. Large cracks were formed in the crust, and layers of rock were thrust over one another, creating thrust faults.

The former seabed was raised up, and in places, those deposits eroded away. In what is now Northern Santa Fe County, Cretaceous rocks were stripped away by erosion. University of New Mexico earth science professor Gary Smith said Cretaceous rocks disappear from the subsurface as you travel north, between Cerrillos and the state penitentiary.

Eventually, by Oligocene time, about 30 million years ago, tectonic forces reversed and extension of the crust began. The Rio Grande Rift began forming in an area that is now southern Colorado down to Chihuahua, Mexico.

More cracks in the earth occurred, and great blocks were down-dropped along normal faults as the crust was pulled apart.

Both the normal and thrust faults complicate the geology of the area for oil exploration. A promising layer could be cut off, or repeated, by these cracks and past movement in the subsurface.

The Rio Grande Rift is divided into basins, divided by faults and buried by the topography of pre-rift time. In places, the basins are filled with thousands of feet of sediment eroded from the mountains.

The Albuquerque Basin is the largest; the Española Basin is one of the next largest.

The Española Basin, which includes the Galisteo area, is tilted to the west. UNM professor Smith likened it to a trap door, hinged downwards. The hinge is along the Sangre de Cristo Mountains. The fault, and therefore the deepest part of the basin where most sediment has collected, is near Los Alamos.

The Albuquerque Basin is tilted to the east, with its hinge near Rio Rancho, and the fault at the foot of the Sandia Mountains.

These two basins meet at La Bajada. The rocks are offset there, causing a topographical separation between the two basins.

Contact Robin Martin at rmartin@sfnewmexican.com.