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416 records · Page 24Linked to original sources
USGS2009

Recent faulting in the Gulf of Santa Catalina: San Diego to Dana Point

We interpret seismic-reflection profiles to determine the location and offset mode of Quaternary offshore faults beneath the Gulf of Santa Catalina in the inner California Continental Borderland. These faults are primarily northwest-trending, right-lateral, strike-slip faults, and are in the offshore Rose Canyon-Newport-Inglewood, Coronado Bank, Palos Verdes, and San Diego Trough fault zones. In addition we describe a suite of faults imaged at the base of the continental slope between Dana Point and Del Mar, California. Our new interpretations are based on high-resolution, multichannel seismic (MCS), as well as very high resolution Huntec and GeoPulse seismic-reflection profiles collected by the U.S. Geological Survey from 1998 to 2000 and MCS data collected by WesternGeco in 1975 and 1981, which have recently been made publicly available. Between La Jolla and Newport Beach, California, the Rose Canyon and Newport-Inglewood fault zones are multistranded and generally underlie the shelf break. The Rose Canyon fault zone has a more northerly strike; a left bend in the fault zone is required to connect with the Newport-Inglewood fault zone. A prominent active anticline at mid-slope depths (300-400 m) is imaged seaward of where the Rose Canyon fault zone merges with the Newport-Inglewood fault zone. The Coronado Bank fault zone is a steeply dipping, northwest-trending zone consisting of multiple strands that are imaged from south of the U.S.-Mexico border to offshore of San Mateo Point. South of the La Jolla fan valley, the Coronado Bank fault zone is primarily transtensional; this section of the fault zone ends at the La Jolla fan valley in a series of horsetail splays. The northern section of the Coronado Bank fault zone is less well developed. North of the La Jolla fan valley, the Coronado Bank fault zone forms a positive flower structure that can be mapped at least as far north as Oceanside, a distance of ??35 km. However, north of Oceanside, the Coronado Bank fault zone is more discontinuous and in places has no strong physiographic expression. The San Diego Trough fault zone consists of one or two well-defined linear fault strands that cut through the center of the San Diego Trough and strike N30??W. North of the La Jolla fan valley, this fault zone steps to the west and is composed of up to four fault strands. At the base of the continental slope, faults that show recency of movement include the San Onofre fault and reverse, oblique-slip faulting associated with the San Mateo and Carlsbad faults. In addition, the low-angle Oceanside detachment fault is imaged beneath much of the continental slope, although reflectors associated with the detachment are more prominent in the area directly offshore of San Mateo Point. North of San Mateo Point, the Oceanside fault is imaged as a northeast-dipping detachment surface with prominent folds deforming hanging-wall strata. South of San Mateo point, reflectors associated with the Oceanside detachment are often discontinuous with variable dip as imaged in WesternGeco MCS data. Recent motion along the Oceanside detachment as a reactivated thrust fault appears to be limited primarily to the area between Dana and San Mateo Points. Farther south, offshore of Carlsbad, an additional area of folding associated with the Carlsbad fault also is imaged near the base of the slope. These folds coincide with the intersection of a narrow subsurface ridge that trends at a high angle to and intersects the base of the continental slope. The complex pattern of faulting observed along the base of the continental slope associated with the San Mateo, San Onofre, and Carlsbad fault zones may be the result of block rotation. We propose that the clockwise rotation of a small crustal block between the Newport-Inglewood-Rose Canyon and Coronado Bank fault zones accounts for the localized enhanced folding along the Gulf of Santa Catalina margin. Prominent subsurface basement ridges imaged offshore of Dana Point m

Special Paper of the Geological Society of America↗
USGS1936

Supplementary report on the ground-water supplies of the Atlantic City region

This report is the second progress report on the ground-water investigations in the Atlantic City region. Many important problems still remain to be solved, however, and it is in no sense a final report. The report covers the area immediately surrounding Atlantic City, extending from Brigantine to Sea Isle City along the coast and from Absecon to Somers Point on the mainland. In addition to this, a few data are presented bearing on the area along the coast as far south as Wildwood. The area lies in the southern part of the New Jersey Coastal Plain, and the water-bearing formations considered are all unconsolidated and of Miocene or more recent age. The major formations in the region dip gently toward the ocean and possibly extend out under the ocean to the edge of the Continental Shelf, about 100 miles from Atlantic City. The principal ground-water supplies in the area are derived from the so-called "800-foot sand," a member of the Kirkwood formation, and from the overlying Cohansey sands. The 800-foot sand is of wide extent and apparently fairly uniform. The Cohansey sands, on the other hand, cover a wide area but are by no means uniform. At the Atlantic City Water Works two Cohansey sands are recognized-the so-called "100-foot" and "200-foot" sands. Neither of these sands can be differentiated from the other sands of the Cohansey formation over a distance greater than 4 or 5 miles in any direction. In addition to the supplies derived from ground water, some surface water is used at present by two of the public water supplies. The quality of the water from all the sources of supply now used is satisfactory. The total consumption of water in the region has increased gradually over the entire period of record, except for a moderate decline from 1929 to 1934. Additional water supplies can be obtained from either of two fairly large streams near the region, and possibly also from a more widespread development of the Cohansey sands. The 800-foot sand should not be counted upon as a source of additional water supply, in view of its liability to salt-water contamination. The same danger also exists in the Cohansey sands on the mainland near the shore, but farther inland it is not a serious menace to the supply from these sands The 100-foot sand at the Atlantic City Water Work has been overdeveloped since 1930, with the result that the head of the water in it has been lowered materially and salt water has been drawn into it through holes in the overlying clay beneath the nearby salt marshes. Three of the five new large-capacity wells drilled to this sand in 1930 have been temporarily or permanently abandoned on account of salt-water intrusion, and the two others will probably have to be abandoned also, unless suitable remedial measures are promptly adopted. It is recommended that the wells to the 100-foot sand be used only when needed to supply the seasonal peak demand and that consideration be given to a project to transform the tidal marshes into a fresh-water pond by means of a suitable dam in order to protect the formation from further contamination. If detailed study proves that the dam and fresh-water pond would not be economically justified, a smaller pond and an embankment and tide gates on the main stream to keep the salt water from flooding the marshes are recommended as less effective but less expensive remedial measures. In view of the experience with the 100-foot sand at the Atlantic City Water Works, it is further recommended that any additional development of the Cohansey sands be preceded by a comprehensive test-well program that will indicate not only the capacity of the sands, but the location of salt water in them and the possibility of its being drawn into existing wells or the proposed new wells. The salt-water intrusion into the 100-foot sand was effectively studied by means of driven-well points, which, it was found, could easily be driven to a depth of about 100 feet. The fact that the screen of these wells was driven with the casing and that no water was used in the drilling process made it possible to collect true samples of water from every sand encountered in them. This, in turn, made it possible to study the vertical distribution of salt water in each well. The interpretation of the vertical distribution of salt water in these wells was very helpful in arriving at a final decision as to the source of the salt water. The 200-foot sand at the Atlantic City Water Works has also been subjected to a considerably increased draft since 1930. Although there is no evidence at present that this sand has been overdeveloped, a study of its characteristics suggests that it may not be capable of yielding permanently the capacity of the present wells that tap it. Three test wells have been installed between the well field and the source from which this sand might derive salt water, and they should be sampled regularly to determine the danger of salt-water intrusion into the sand. In a landward direction this sand merges into the other Cohansey sands. It is therefore advisable that any additional development of the Cohansey sands should be undertaken so far inland that the pumping from it will not affect the present wells to the 200-foot sand and thereby increase the danger of salt-water contamination in them. At present more potable water is taken from the Atlantic City 800-foot sand than from any other source of supply for the region. This sand is the sole source for some of the smaller communities on the barrier beaches. The original static head of the water in it at Atlantic City was between 20 and 25 feet above sea level. The head has been lowered more than 50 feet over much of the region, and in parts of Atlantic City it has been lowered considerably more than 100 feet. A consideration of the principles governing the relation between salt water and fresh water in water-bearing sands indicates that the 800-foot sand probably contained salt water at a distance of 5 or 10 miles out from Atlantic City before any water was pumped from it. The evidence collected in this investigation indicates that the cone of depression created by the pumping from this sand in the Atlantic City region has probably extended inland to the intake area of the sand, the nearest part of which is probably about 40 miles from Atlantic City. If this is so, the conclusion is almost inescapable that it has also extended oceanward for a distance considerably greater than the 5 or 10 miles to the original zone of contact between the fresh and salt waters, and that salt water is probably being drawn toward the Atlantic City region through this sand. The time of its arrival will depend primarily upon the rate of pumping in the region and upon how much of the fresh water that originally lay between the region and the zone of contact must be removed before the salt water can reach the region. It may arrive in the near future if it advances in the form of a narrow tongue. On the other hand, if it advances along a broader front; so that more of the intervening fresh water must be pumped out of the formation, its arrival may be delayed for some time.

New Jersey↗