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Assessing Offshore Marine Sand Deposits with Probabilistic Models
Mapping the distribution of mineral resources of the United States has been a central element of the U.S. Geological Survey (USGS)'s research mission since the agency was established by the Organic Act of 1879, which called for "the classification of the public lands and examination of the geological structure, mineral resources, and products of the national domain." Although much of the effort over the past 130 years has gone into studying and mapping terrestrial regions, most U.S. territory extends offshore across the continental margins, 200 mi (360 km) to the seaward limits of the Nation's Exclusive Economic Zone (EEZ), designated in 1983. These regions on the continental margin have been little mapped or studied, and basic information—such as geologic maps of the sea floor, understanding of the nature and distribution of sedimentary deposits, and mineral-resource assessments—is sparse.
The submerged continental margins are the product of the underlying geology and dynamic oceanographic processes, particularly processes associated with the late Quaternary marine transgression, during which sea level rose about 120 m over the past 21,000 years since the Last Glacial Maximum. Taken together, these drowned regions of the U.S. EEZ are larger than the continental United States, and they contain submerged landforms that provide various functions and benefits. Some parts of the continental margins also contain unconsolidated mineral deposits, such as sand and gravel, that are potential aggregate resources. Coastal erosion caused by a combination of natural processes (such as storms, sea-level rise, and land subsidence) and human activities (such as the building of dams and levees that reduce sediment input to coastal systems) is a problem for all coastal States. Population and development in the coastal zone continue to increase, placing more people and infrastructure at risk in vulnerable coastal areas. As predicted climate change accelerates global sea-level rise and increases storminess, coastal regions will undergo even greater erosion and storm-surge flooding in the near future. A practice used to offset the negative impacts of coastal erosion in certain areas is beach nourishment, a method of dredging sand from offshore areas and pumping it ashore to widen and elevate the beach and dune. Beach nourishment is viewed for many developed coasts as a cost-effective and environmentally acceptable short-term (decades of protection) method for mitigating coastal erosion, reducing storm and flooding risk, and restoring degraded coastal ecosystems. For beach nourishment to be successful, however, large volumes of high-quality sand are necessary. For project benefits to exceed costs, the sand deposits must be located reasonably close to the beaches being considered for nourishment.
Sand bodies on the inner continental shelf are commonly deemed desirable sources for beach nourishment because of the quality of the sand and its proximity to the coast. Sand shoals off headlands, sand deposits in drowned stream channels, and sand deposits at the edge of drowned shorelines are a few examples of potential "borrow areas," where sand could be dredged for use in beach nourishment. Demand for offshore sand and gravel is likely to increase over the next 50 years as accelerated sea-level rise and increased storminess caused by climate change increase coastal erosion and flooding hazards. Demand for offshore marine aggregates might also increase as onshore supplies of aggregate are depleted in some parts of the country. Turning to offshore marine aggregates, however, is not a simple solution. The sea-floor areas containing sand bodies commonly also host important benthic habitats that are damaged by dredging; and for many U.S. coastal areas, the offshore sand resources appear to be inadequate to meet longer-term needs for beach nourishment and coastal protection. To help meet societal needs, the USGS Marine Aggregates Resources and Processes (MARP) Project focuses on characterizing and mapping offshore sediment and developing statistical models and techniques for assessing marine sand-and-gravel resources. One project task involves gathering a wide variety of marine geologic data into the usSEABED system. This system provides a centralized, fully integrated digital database of marine geologic data collected over the past 50 years by the USGS, other Federal and State agencies, universities, and private companies. To date, approximately 400,000 data points from the U.S. EEZ reside in the usSEABED system, which combines a broad array of physical data and information (both analytical and descriptive) about the sea floor, including sediment textural, statistical, geochemical, geophysical, and compositional information. Data from usSEABED are available to the marine community through USGS Data Series publications. Already published are USGS Data Series reports for three regions: offshore the Atlantic coast (DS 118), the Gulf of Mexico and Caribbean (DS 146), and the Pacific coast region (DS 182). Updates of Data Series DS 118 and DS 146 with additional data sets are expected in summer 2009; similar Data Series reports for Hawai‘i and Alaska are in preparation. Another project task deals with modeling and assessing offshore deposits. Two reports describing the development of a probabilistic model and its application to the New York-New Jersey continental-shelf region were published in USGS Bulletin 2209 this year. The model presented in chapter M (Bliss, J.D., Williams, S.J., and Bohm, K.S., 2009) is believed to be the first such statistical model developed for marine sand bodies by using a three-part mineral-resource-assessment approach to an offshore region. In chapter N (Bliss, J.D., Williams, S.J., and Arsenault, M.A., 2009), the authors conclude that an estimated 3.9 billion m3 of marine sand resources is present in marine sand deposits associated with capes and sand ridges in three representative regions, or tracts, on the continental shelf off New York and New Jersey. Admittedly, only part of these probable resources are actually available for production, depending largely on environmental, economic, preemptive-use, and political factors. These estimates, derived from probabilistic distributions of sand resources, were made by using the deposit models and Monte Carlo simulation techniques described in Chapter M. The estimated sand-resource volumes discussed in chapter N are for only three tracts and just for the Holocene sand resources contained in cape- and ridge-associated marine sand deposits. Other areas may qualify as tracts for this deposit type on the Atlantic continental shelf off New Jersey and New York but are not delineated or addressed in this initial evaluation. Marine sand also occurs in other deposit types of different geologic ages (for example, paleostream channels, blanket and outwash deposits, ebb-tidal shoals, and paleoshorelines and deltas formed at times of lower sea level), which are present on the Atlantic continental shelf off New Jersey and New York but were not considered in this assessment. Results of this modeling and assessment of marine sand resources are expected to be used by Federal and State resource and regulatory agencies in carrying out their missions. The MARP plan is to apply the Monte Carlo simulation technique and the three-part assessment approach to other areas on the Atlantic coast margin, and to produce a suite of online maps based on usSEABED showing sea-floor-sediment characteristics and distribution. The initial maps for the Atlantic continental shelf and the northern Gulf of Mexico will be published in late 2009. Full references for the two recent reports on modeling and assessing offshore deposits are: Bliss, J.D., Williams, S.J., and Bohm, K.S., 2009, Modeling cape- and ridge-associated marine sand deposits; a focus on the U.S. Atlantic continental shelf, chap. M of Bliss, J.D., Moyle, P.R., and Long, K.R., eds., Contributions to industrial-mineral research: U.S. Geological Survey Bulletin 2209-M, 28 p. [URL http://pubs.usgs.gov/bul/b2209-m/]. Bliss, J.D., Williams, S.J., and Arsenault, M.A., 2009, Mineral resource assessment of marine sand resources in cape- and ridge-associated deposits in three tracts, New York and New Jersey, United States Atlantic continental shelf,chap. N of Bliss, J.D., Moyle, P.R., and Long, K.R., eds., Contributions to industrial-minerals research: U.S. Geological Survey Bulletin 2209-N, 6 p. [URL http://pubs.usgs.gov/bul/b2209-n/].
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in this issue:
Significant Gas Resource Discovered in Gulf of Mexico
USGS: Your Resource During Hurricane Season |
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U. S. Department of the Interior | U.S. Geological Survey
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