VOYAGER MISSION RESULTS

The arrival of the Voyager spacecraft at the Jupiter system marked an explosive expansion in the field of planetary geology. The Geological Survey provided scientific expertise in planning the mission, in designing the instruments, and in interpreting the mission results. The imaging cameras aboard Voyager 1 and 2 provided the world with the first closeup images and measurements of not only Jupiter and its newly discovered rings, but also of four new worlds comparable in size and physical properties to the Moon, Mars, Mercury, Venus, and Earth. This new information doubled the number of planetary bodies with which the Earth's physical characteristics, geologic processes, and history can be compared. One of the most intriguing aspects of the mission proved to be the astounding complexity and diversity of the four large Galilean satellites, lo, Europa, Ganymede, and Callisto.

The most exciting and geologically important finding was the discovery of active volcanism on lo. lo, innermost of the four large Galilean satellites, is very nearly the same size and density as our Moon. The complete absence of detectable impact craters on lo demonstrates that its surface is being continuously renewed by these active volcanic processes, which are believed to be sustained by tidal heating. Eight active volcanoes have been positively identified; many of these possess plumes that extend up to 150 miles above the surface. Io has the greatest surface relief of the Galilean satellites; this relief is in the form of calderas, scarps, and isolated mountains that rise above the landscape. A variety of studies are now underway, ranging from geologic mapping to thermodynamic studies of the volcanoes now thought to be erupting sulfur and sulfur dioxide.

Only limited images of Europa were obtained with resolutions of about 2.5 miles. Europa, also the size of our Moon, is slightly less dense and is thought to have a crust of ice, perhaps a frozen "ocean," that is 60 miles deep. Voyager 2 images revealed a highly reflective surface covered with numerous dark intersecting linear features that were visible at the limit of resolution. There are many theories as to the origin of the pattern, and it is generally believed that it is caused by crustal expansion due to freezing. Only three impact craters have been confidently identified, which suggests that the freezing

episode took place after the early postaccretional meteorite impact bombardment, which formed densely cratered highlands on the Moon, Mars, and Mercury, had tapered off.

Ganymede and Callisto, the largest Galilean moons, are about the diameter of Mercury but have low densities, suggesting these exteriors are about one-half water or water ice. Two major types of terrain on Ganymede, dark cratered and grooved, are evident and continuous over the entire surface of the satellite. The grooved terrain is younger and consists of a complex network of parallel ridges and troughs, which occur in various sized segments, that divides the older heavily dark cratered terrain into isolated polygons. It appears that the dark-cratered terrain was formed after the heavy bombardment. Crater densities on the grooved terrain are extremely variable, suggesting that the grooves formed early in Ganymede's history, lasted for a long period of time (perhaps several hundred million years), and have since been inactive. Voyager 2 revealed an additional surprise in the form of subparallel concentric ring structures in various parts of the ancient darkcratered terrain. These rings are thought to be the remnants of a huge ancient impact basin, most of which has subsequently been broken apart by tectonic activity.

Callisto, the outermost of the large Galilean satellites, is probably the most densely cratered object yet seen in the solar system. Three large concentric ring structures, vestiges of enormous impact basins, are the only prominent features that interrupt the sea of craters. The complete absence of any large craters or measurable relief is indicative of viscous flow of an icy crust. Callisto's surface probably corresponds to the dark terrain of neighboring Ganymede.

Information gained from the Voyager Missions has greatly compounded the sum of knowledge concerning the evolution of the solar system. Detailed observations of the Galilean satellites have provided a base for studying other planetary objects of similar size and geologic history, such as the Earth and other terrestrial planets of the inner solar system. Both spacecraft have since left the Jupiter system, and equally successful encounters with Saturn are anticipated in 1980 and 1981. The U.S. Geological Survey will also play a major role in the investigation of the large satellites of Saturn.

THE ATLANTIC REEF

A promising area for future oil and gas exploration has been identified seaward of the OCS off the Atlantic Coast. The prospective area is what appears to be an extensive ancient buried reef, similar to the Great Barrier Reef that parallels the coast of Australia for over a thousand miles.

Reefs are regarded as potentially good reservoirs for oil and gas because they may be very porous and normally form in proximity to deep basins containing organic rich sedimentary rocks suitable for the generation of oil and gas.

For nearly a decade, geologists in industry, academia, and government have speculated about the presence of a buried reef off the Atlantic Coast. Following the oil crisis of 1973, the U.S. Geological Survey undertook a systematic regional geophysical survey of the OCS as an initial step in an offshore resource-assessment program. The geophysical data were used to delineate the major sedimentary basins on the OCS. The data also show an anomalous structure that has the characteristics of a reef. However, evidence supporting this interpretation has become available only recently. This evidence was obtained by using deep submersibles to sample rock exposed on the seafloor of the Continental Margin and from the drilling of an off-structure stratigraphic test well (COST B-3 well) by industry.

It appears that the buried ancient reef can be traced intermittently through the northern Gulf of Mexico, to the great petroliferous reef complex of the Golden Lane, and possibly even to the Reforma (site of a recent oil and gas discovery) and Campeche shelf provinces of Mexico. The reef is almost continuous along the Atlantic Margin from Maine to Florida. It occurs in water depths ranging from about 2,000 to 6,000 feet and is covered by 6,000 feet of sedimentary rocks. The reef is roughly 10,000 to 20,000 feet thick and 15 miles wide. It is bordered by thick layers of potential source rocks and appears to have a suitable seal above it to trap accumulated oil and gas. It is developed best along the seaward edge of the Baltimore Canyon Trough and, in fact, forms the eastern edge of this basin.

The core of the reef is limestone, possibly formed by the skeletons of growing fauna and flora, or carbonate sands. The forereef area may be characterized by coarse reefal debris that has broken off the core and fallen into deeper water. The ends of the forereef interfinger with the basin rocks that may be the source rocks of

petroleum. The backreef rocks are probably limestones that grade into sands and muds derived from the land. Much of the Baltimore Canyon Trough is filled with these kinds of sands and muds.

The COST B-3 well drilled by industry encountered limestones with interbedded sands containing gas. These limestones are thought to be backreef rocks. The Survey has evaluated the petroleum potential of the reef complex. Estimates of resources in undrilled regions are made by comparing their geology and geologic history with those of similar regions that have been drilled-not all reefs make good reservoirs. Those reefs that have been raised above sea level prior to final burial and exposed through the leaching action of fresh water exhibit greatly enhanced reservoir characteristics such as porosity and permeability. Estimation of the chances that porosity enhancement has taken place is critical in the resource assessment. For this purpose, we compared the Atlantic reef system with Texas backreef production, reflecting those limestones that had not undergone porosity enhancement, and also with the highly productive Mexican reefs and the reefs of the Permian Basin of west Texas, where the leaching action of the fresh water has suitably enhanced the porosity. By using such comparative techniques, the Survey has estimated that the reef complex in the mid-Atlantic lease area may contain 1 to 6 billion barrels of recoverable oil and the entire Atlantic reef trend may contain 2 to 15 billion barrels of oil. In both cases, the assessment assumes that oil is present. However, if the amount of organic material in the source rocks is inadequate, or is of the wrong type, or was never exposed to sufficiently high temperatures, there may be no oil at all. There are many other factors that bear on whether or not oil will be preserved in the rocks.

Because of these assessments, BLM has extended the area boundary on the Call for Nominations in the forthcoming Sale 59 in the mid-Atlantic area to include the reef complex. The maximum water depth in this call area is about 8,000 feet. Industry has the capability to explore in about 6,000 feet of water, but current production is limited to a depth of about 1,000 feet. However, production systems capable of operating in much greater water depths have been built and tested in anticipation of economic discoveries in deep water.

The geologic investigations and studies described above contribute to the overall effort to discover new areas favorable for oil and gas exploration.

Jurassic-Cretaceous reef trend.