First State GeologyCurrent information about Delaware‘s geology, hydrology, and mineral resources

Published twice yearly by the Delaware Geological SurveyUniversity of Delaware

Vol. 19, No. 1 • Winter 2001

New Piedmont BedrockGeologic Map andReportBy William S. Schenck

Two new Delaware Geological Survey(DGS) publications describe the geology ofthe Piedmont of Delaware and a portion ofPennsylvania which is included to show theextent of the Mill Creek Nappe and theArden Plutonic Supersuite. DGS GeologicMap Series No. 10 (scale 1:36,000),“Bedrock Geologic Map of the Piedmont ofDelaware and Adjacent Pennsylvania” is byWilliam S. Schenck, Margaret O. Plank(both DGS), and LeeAnn Srogi (WestChester University). It is included as Plate 1in the rear cover pocket of DGS Report ofInvestigations No. 59 (52 p.), “BedrockGeology of the Piedmont of Delaware andAdjacent Pennsylvania” by Margaret O.Plank, William S. Schenck, and LeeAnnSrogi.

The information provided by the mapand report is important not only to geologistsbut also to hydrologists who wish to under-stand the distribution of water resources, toengineers who need bedrock informationduring construction of roads and buildings,to government officials and agencies whoare planning for residential and commercialgrowth, to educators and students, and to cit-izens who are curious about the bedrockunder their homes.

The Appalachian Piedmont Province ofDelaware and adjacent Pennsylvania recordsthe collision of a magmatic arc with a conti-nental landmass and adjacent forearc basinsediments during the early Paleozoic Era.As a result of this collision, the rocks areintensely deformed and metamorphosed tothe amphibolite and granulite facies. Theoldest rocks are Grenville-age layeredgneiss, migmatite, and amphibolite that arecorrelated with the Baltimore Gneiss ofMaryland. The Grenville-age rocks areunconformably overlain by the LateProterozoic to early Paleozoic SettersFormation and the Cockeysville Marble of

the Glenarm Group. These three rock unitsare exposed in the Mill Creek Nappe.

The Wissahickon Formation represents athick sequence of sediments deposited in aforearc basin that are now metamorphosedand in thrust contact with the rocks of theMill Creek Nappe. Much of theWissahickon Formation shows repetition oflayers characteristic of distal turbidites.Included withinthe metamor-phosed sedi-ments are anultramafic-maficlens, a thin dia-base dike, andtwo types ofamphibolites,one with traceelement chem-istry characteris-tic of basaltsformed at aseafloor-spread-ing center, andthe other withtrace elementchemistry char-acteristic ofintraplatebasalts.

Combined detailed mapping, petrogra-phy, geochemistry, and U-Pb geochronologyhave resulted in the redefinition of two rockunits and formal recognition of eleven newunits. The new units occur within the rede-fined Wilmington Complex and are classi-fied as lithodemes. Lithodemic stratigraphyallows for mapping individual rock unitswithin the Wilmington Complex without acomplete understanding of the complex rela-tionships that exist between them. TheWilmington Complex includes rocks associ-ated with the development of a Paleozoicmagmatic arc: five metaplutonic lithodemesare the Brandywine Blue Gneiss,Montchanin Metagabbro, Mill CreekMetagabbro, Barley Mill Gneiss, andChristianstead Gneiss; three igneous plutonsare the Bringhurst Gabbro, Iron Hill Gabbro,and the redefined Arden Plutonic Supersuite

comprising the Ardentown Granitic Suite,Perkins Run Gabbronorite Suite, and onelithodeme of biotite tonalite; and threemetavolcanic units named the Rockford ParkGneiss, Windy Hills Gneiss, and FaulklandGneiss.

John N. Aleinikoff of the U. S.Geological Survey determined U-Pb ages onzircons found in seven rock units. They are

434�4 Ma for the Arden Supersuite, 476�6Ma for the Brandywine Blue Gneiss, 476�4Ma for the Rockford Park Gneiss, 482�4Ma for the Faulkland Gneiss, 481�4 Ma forthe Windy Hills Gneiss, 470�9 Ma for theBarley Mill Gneiss, and 488�8 Ma for theChristianstead Gneiss.

Geochemical analyses of the mafic rocksprovided a breakthrough in efforts to subdi-vide units within the Wilmington Complex.Samples could be grouped by their trace ele-ment concentrations and the groups identi-fied spatially in the field. A detailed reporton the geochemistry of these rocks will bepublished by Plank et al. (in press) as DGSReport of Investigations No. 60.

The geologic map, report, and all datafor outcrops used for this study are accessi-ble through the DGS web site atwww.udel.edu/dgs/webpubl.html.

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New Report onPesticides in Delaware’sShallow Ground WaterBy Scott C. Blaier and Stefanie J. Baxter

A federal regulation proposed in 1996,but not yet implemented, by the U.S.Environmental Protection Agency (USEPA)is to require individual states to developPesticide Management Plans (PMPs) to pro-tect ground water from contamination by pes-ticides. The regulation designates the triazineherbicides atrazine, simazine, and cyanazine,and the acetanilide herbicides alachlor andmetolachlor as the first five pesticides thatwould require a PMP. All five are used pri-marily for weed control on agricultural crops.The Delaware Department of Agriculture(DDA) is the state agency responsible fordevelopment and implementation of thePMPs.

Ground-water monitoring is required foreach PMP and offers two important benefits:(1) it identifies areas where pesticides may beleaching into ground water, and (2) it is ameans for evaluating and measuring the suc-cess of management strategy over time. In1995, through a cooperative effort betweenthe two agencies and supported in part by theUSEPA, the DDA and DGS developed amonitoring strategy to support pendingPMPs. This included the design and imple-mentation of a shallow ground-water networkfor monitoring pesticides by sampling waterfrom domestic, agricultural, and dedicatedmonitoring wells located throughoutDelaware south of U.S. Route 40. FromOctober 1995 to September 1998 approxi-mately 350 ground-water samples from 136wells were collected and analyzed for the fivepesticides designated by the USEPA. Alsoanalyzed, but less extensively, were the herbi-cides 2,4-D and metribuzin and the insecti-cides carbofuran and chlorpyrifos.

The results of this study are now avail-able in DGS Report of Investigations No. 61(23 p.), “The Occurrence and Distribution ofSeveral Agricultural Pesticides in Delaware’sShallow Ground Water” by Scott C. Blaier(DDA) and Stefanie J. Baxter (DGS).

The purpose of the report is to answerthree questions: (1) Are agricultural pesticidespresent in Delaware’s shallow ground water?(2) Which pesticides are present and at whatconcentrations? (3) Where are the pesticidesbeing detected? Data collected as part of thiscontinuing monitoring program will be usedto support development of PesticideManagement Plans for Delaware.

Of the 1,725 analyses performed, 96 per-cent had concentrations less than 1.0 g/l.This includes almost 80 percent of the sam-ples that showed no measurable concentra-tions of the pesticides studied. Although

some analyses indicated the presence of apesticide, confirmatory tests showed that alldomestic wells meet state and federal drink-ing water standards.

The new publication is available from theDelaware Geological Survey offices at theUniversity of Delaware and can be obtainedby calling (302) 831-2833 or via email at delgeosurvey@udel.edu, or from theDelaware Department of AgriculturePesticides Section at 1-800-282-8685. Thereport is also available on the DelawareGeological Survey web site at:http://www.udel.edu/dgs/webpubl.html.Continuously updated monitoring data for themonitoring wells can be found at theDelaware Department of Agriculture GIS website at: http://smartmap.com/dda/.

Two New Publicationson Earthquakes

Two publications on earthquakes, both byStefanie J. Baxter, are now available, but onlyonline, on the DGS web site athttp://www.udel.edu/dgs/webpubl.html.“Earthquake Basics,” DGS SpecialPublication No. 23, provides a brief overviewof the causes of earthquakes, how they aremeasured, a glossary of earthquake terminol-ogy, and tables noting the largest U.S. earth-quakes. Illustrations include, among others, amap of Earth’s major tectonic plates, mido-cean ridges, and trenches; the nature ofground motion produced by four types ofseismic waves; three primary types of motionalong faults; and a contoured intensity map ofthe 1973 3.8-magnitude earthquake centerednear Wilmington.

“Catalog of Earthquakes in Delaware,”DGS Open File Report No. 42, lists andshows with a map the earthquake history ofDelaware since 1871. Since that date, 69earthquakes have been documented or sus-pected in Delaware. Since 1677, more than550 earthquakes have been documented with-in a radius of 150 miles from Delaware. In1997, the U.S. Geological Survey (USGS)and the Federal Emergency ManagementAgency (FEMA) reclassified Delaware froma low to a medium seismic risk.

Included in the publication is a brief his-tory of the DGS seismic network of short-period vertical seismometers to detect localearthquake activity. Now there are five suchstations operating in the state. Another sta-tion near Greenwood is operated by theUSGS and is maintained by the DGS. It ispart of the U.S. National SeismographNetwork and is designed to record the hori-zontal and vertical earth movements causedby earthquakes of greater than 2.5 magnitudethat occur within a 200-mile radius.

Geophysical LoggingOffers A Look AtDelaware’s SubsurfaceGeologyBy Peter P. McLaughlin and Scott A. Strohmeier

Investigations of the subsurface geologyand groundwater resources of Delaware dependheavily on subsurface data obtained from wellsand boreholes. In many cases, the rock unitspenetrated are known only from samplesobtained during drilling. Another method isgeophysical logging which allows us to obtain acontinuous, high-resolution record of the physi-cal properties of the rocks and their containedfluids. From this we can interpret the variousrock types that were drilled.

In geophysical logging, a probe designed tomeasure certain physical properties, such aselectrical conductivity, natural radiation, or tem-perature, is lowered and raised in the boreholeby means of a steel cable spooled from a winch.Readings from the sensors on the probe aretransmitted electrically through the cable to adata collection unit.

The DGS maintains a geophysical loggingprogram as part of its responsibility for system-atic investigation of the geology and waterresources of Delaware. Our collection of geo-physical logs dates to the late 1950s and cur-rently totals more than 1400 logs from over1000 wells. In 1998, we upgraded our loggingcapabilities with the acquisition of a new,portable digital logging system capable of log-ging to a depth of nearly 1000 ft.

The system includes a suite of four probes.The natural gamma tool measures naturalgamma radiation, which can be used as an indi-cator of the relative content of sand versus clay.Quartz sands are typically distinguished ongamma logs by low values, whereas clay-richsediments have higher gamma values thatreflect the presence of trace amounts of radioac-tive elements such as uranium, thorium, andpotassium-40 (see figure). This tool is the mostversatile part of our logging suite and can berun in almost any hole of 2-inch-diameter orgreater. The multi-parameter electric-log tool isused to obtain electric logs in uncased bore-holes, and simultaneously records five differentresistivity logs plus spontaneous potential andnatural gamma. The resistivity logs stronglyreflect the fluid content of the formation, withfresh-water-bearing intervals such as aquifersands producing a high-resistivity signal in con-trast with low-resistivity clay zones. The mag-netic induction tool produces conductivity logswith some similarity to resistivity logs but canbe used in PVC-cased wells. Rather thandirectly measuring the electrical properties inthe borehole, this tool measures them indirectlyby inducing an electro-magnetic response inthe surrounding formation. The three-arm

First State Geology Winter 2001• 3

caliper tool measures the diameter of the holeto help understand drilling conditions andprovide for correction of log measurements.

These tools provide significant benefits togeologic and hyrdologic investigations. Forexample, geophysical logs are important inevaluating aquifers. When drilling a waterwell, drilling characteristics and drill cuttingsare usually used to recognize where anaquifer has been penetrated. However, thesecriteria are not always definitive, and in prob-lematic cases geophysical logs can help iden-tify the most promising intervals to screen for

water production. Because certain geophysi-cal logs reflect the nature of the pore fluids ina formation, they are also useful in coastalareas to evaluate cases of potential salt-waterintrusion of aquifers. In addition, geophysi-cal logs are an integral part of subsurfacestratigraphic studies and geological mappingprojects and are extensively utilized in areascurrently being mapped in central SussexCounty. Recognizing the broad utility ofgeophysical logging, the DGS strives to workwith water well drillers as well as other local,state, and federal agencies to coordinate col-lection and sharing of geophysical logs inareas that further our understanding of thegeology of Delaware and at the same timeprovide data beneficial to others’ projects.

Earth Science Week andCoast Day PostersAvailable Online

Owing to their popularity, the supplies oftwo DGS posters, one for Earth Science

Week 2000 and the other for Coast Day 2000,have been depleted. They are, however,available online.

More than 1500 Earth Science Weekposters were distributed to schools and schoolchildren and the public who attended activi-ties held at the Delaware Geological Survey.The poster, “Selected Geomorphic Featuresof Delaware” by Kelvin W. Ramsey, WilliamS. Schenck, and Lillian T. Wang (DGSSpecial Publication No. 24), shows two mapsside by side. One is a black, white, and graytone, 3-D shaded relief image of Delaware.At a scale of 1 inch to 4 miles, the map wascreated using digital elevation data, andshows the coastal marshes, rivers, andstreams plotted in blue on top of the image.On the accompanying map at the same scaleand labeled with cities and towns, selectedgeomorphic features visible on the shadedrelief image are identified including the majordrainage divide, ancient Delaware bay andAtlantic coastlines, Carolina bays, sanddunes, an area called “The Levels,” Iron andChestnut hills, Piedmont uplands, CoastalPlain, and the Fall Line.

The Coast Day poster, “PhysiographicRegions of the Delaware Atlantic Coast” byKelvin W. Ramsey, William S. Schenck, andLillian T. Wang (DGS Special PublicationNo. 25), is a 1-inch to 1-mile map that iden-tifies the headlands, bay barriers, coastallagoons, and the Cape Henlopen spit complexalong the entire Atlantic coast of Delaware.Other information includes the direction andamount of sediment transport, locations of thepre-1929 inlets and the old Cape HenlopenLighthouse, and a brief history of the Lewesbreakwater. Over 1400 of the posters weredistributed for this year’s College of MarineStudies Coast Day held at the University ofDelaware Campus in Lewes.

Special Publications 24 and 25 are down-loadable from the DGS web page athttp://www.udel.edu/dgs/webpubl.html. Printon demand copies can be furnished at a costof $5.00 each by contacting the DGS at (302) 831-2833 or via email at delgeosurvey@udel.edu.

Coast Day 2000By Lillian Wang

On Sunday, October 1, 2000, TheDelaware Geological Survey sponsored a“Dig for Fossils” at the University ofDelaware, College of Marine Studies 24thannual open house, Coast Day, in Lewes,Delaware. Children dug up and identifiedDelaware-area fossils, and Pete McLaughlinwas on hand to verify identifications. LillianWang distributed 1500 “Coast Day 2000”posters and 400 “Earth Science Week 2000”posters (see article on these publications in

this issue). Plans are to publish new postersannually and to distribute them during CoastDay and Earth Science Week, which are bothin October.

Safety Committee of theMonth

The University of Delaware Departmentof Occupational Health and Safety (DOHS)has recognized the Delaware GeologicalSurvey as Department of the Month forJanuary 2001. The safety committee is ledby Kelvin Ramsey with members RolandBounds, Pete McLaughlin, and Tom Smith.The DOHS citation, online athttp://www.udel.edu/OHS/, recognizes theDGS for its sessions on chemical hygienetraining and right-to-know, use of hydrofluo-ric acid, boating safety, forklift safety, properlifting, Lyme disease, and trenching safety.Also noted was the DGS cell phone use poli-cy that phones are not to be used while driv-ing and that cell phones are to be on handwhenever fieldwork is being done, whichallows persons working in remote locations tohave a way to summon help in the event ofan emergency.

From the DOHS, “We appreciate the timeand effort this group takes to make safetytheir first priority. The professionalismdemonstrated by this group is certainly wor-thy of recognition. Please join me in congrat-ulating DGS for their fine effort!”

Gem and Mineral ShowBy Roland E. Bounds

The Delaware Mineralogical Society willbe holding its annual Earth Science, Gem andMineral Show on March 3 and 4, 2001, at theBrandywine Terrace, 3416 Philadelphia Pike,Claymont, Delaware. Hours are 10 a.m. to 6p.m. on Saturday and 11 a.m. to 5 p.m. onSunday. There will be exhibits, dealers for

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Geophysical and lithologic logs from wellQj32-27 at Bethany Beach, Del. Naturalgamma log values increase overall upward inthis interval, reflecting a lithologic changefrom sand to clayey silt beds.

Pete McLaughlin helping a “budding paleon-tologist” identify fossils.

minerals, fossils, and jewelry, as well as doorprizes, a raffle specimen, and the JuniorBooth for kids items. Parking is free andfood is available on site. There is an entryfee. For more information contact WayneUrion at 302-998-0686.

Publications

Recent DGS Publications

Geologic Map SeriesNo. 10, Bedrock Geologic Map of the

Piedmont of Delaware and AdjacentPennsylvania: William S. Schenck, MargaretO. Plank, and LeeAnn Srogi, 2000, scale1:36,000.

Reports of InvestigationsNo. 59, Bedrock Geology of the Piedmont

of Delaware and Adjacent Pennsylvania:Margaret O. Plank, William S. Schenck, andLeeAnn Srogi, 2000, 52 p.

No. 61, The Occurrence and Distributionof Several Agricultural Pesticides inDelaware’s Shallow Ground Water: Scott C.Blaier and Stefanie J. Baxter, 2000, 23 p.

Special PublicationsNo. 23, Earthquake Basics: Stefanie J.

Baxter, 2000. Available only online athttp://www.udel.edu/dgs/pub/SP23.pdf.

No. 24, Selected Geomorphic Features ofDelaware: Kelvin W. Ramsey, William S.Schenck, and Lillian T. Wang, 2000, map, onesheet, scale 1 inch to 4 miles. Available onlyonline athttp://www.udel.edu/dgs/pub/SP24.pdf.

No. 25, Physiographic Regions of theDelaware Atlantic Coast: Kelvin W. Ramsey,William S. Schenck, and Lillian T. Wang,2000, map, one sheet, scale 1 inch to 1 mile.Available only online athttp://www.udel.edu/dgs/pub/SP25.pdf.

Open File ReportsNo. 42. Catalog of Earthquakes in

Delaware: Stefanie J. Baxter, 2000, 6 p.Available only online athttp://www.udel.edu/dgs/pub/OFR42.pdf.

Other Publications by DGSStaff

Thomas E. McKenna, 2000, Low-Budget, Do It Yourself Thermal-InfraredImaging of Ground-Water Discharge to SurfaceWaters [Abstract]: Geological Society ofAmerica Abstracts with Programs, v. 32, no. 7,p. A60; with William J. Ullman, Kuo-ChuinWong, Joseph R. Scudlark, John A. Madsen,David E. Krantz, and A. Scott. Andres, 2000,CISNet: Nutrient Inputs as a Stressor and NetNutrient Flux as an Indicator of StressResponse in Delaware’s Inland BaysEcosystem [Abstract]: Proceedings of theCoastal Intensive Sites Network (CISNet),STAR Grants 2000 Progress Review, EPAReport # EPA600/R-00/060, EnvironmentalProtection Agency, Washington, D. C., p. 13.

Gerald M. Kauffman, Martin W.Wollaston, and John H. Talley, 2000, TheDrought of 1999: The Efforts of the Governor’sWater Supply Task Force in Delaware:Proceedings of American Water ResourcesAssociation Conference, Miami, p. 249-253.

Staff Notes

PresentationsAt the Geological Society of America

Annual Meeting, Reno, Nevada, Nov. 13:Peter P. McLaughlin, Jr., and Richard N.Benson with Kenneth G. Miller, and James V.Browning (both Rutgers University), “RegionalCorrelation of Miocene Sequences, MiddleAtlantic Coastal Plain: New Results fromBethany Beach, Delaware;” Thomas E.McKenna, “Low-Budget, Do It YourselfThermal-Infrared Imaging of Ground-WaterDischarge to Surface Waters.”

At the Delaware GIS Day 2000Conference, Dover, November 17: William S.

Schenck and Nicole M. Minni (WaterResources Agency), “GIS may Make MapsFast, but Cartography is Still an Art;” andLillian T. Wang and Thomas E. McKenna,“Digital Delineation of Ground-Water Divides,Inland Bays, Sussex County, DE.”

Thomas E. McKenna “ Aerial Thermal-Infrared Imaging and ‘Ground-Truthing’ toQuantify Ground-Water Discharge to a CoastalEstuary on the Atlantic Coast of Delaware,” atDepartment of Geosciences, State University ofNew York, Stony Brook, October 12 ; “TheCISNet Program: Collaborative Research inDelaware’s Inland Bays,” at Coast Day 2000,University of Delaware, College of MarineStudies, Lewes, October 1.

William S. Schenck, “Geology ofDelaware,” at the Archaeological Associationof Delaware monthly meeting, Greenbank Mill,Wilmington, June 21.

Martin W. Wollaston, Gerald M.Kauffman, and John H. Talley, “Report to theGovernor and the State Legislature Regardingthe Progress of the Delaware Water SupplyCoordinating Council,” May 31.

Service and AwardsRobert R. Jordan will receive the

President’s Award of the Division ofEnvironmental Geology (DEG) of theAmerican Association of Petroleum Geologists(AAPG) at the next AAPG convention in June2001. The award is the highest and most dis-tinguished one of the DEG and is given for theadvancement and betterment of the science andprofession of environmental geoscience.Jordan is one of the founding members of theDEG.

John H. Talley received the George V.Cohee Public Service Award from the EasternSection of the American Association ofPetroleum Geologists for “His enthusiasticapplication of geologic science for public bene-fit.”

Congratulations to William S. Schenckwho has completed 20 years service with theDGS and University of Delaware as ofFebruary 16.

Delaware Geological SurveyUniversity of DelawareNewark, DE 19716-7501

First State Geology is published by the DelawareGeological Survey, a State agency established by anAct of the Delaware General Assembly in 1951 andorganized as a unit of the University of Delaware.

Robert R. Jordan State Geologist and DirectorRichard N. Benson,Editor, First State Geology

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