1Map-Scale Structures and Contact RelationshipsGeol341-Structural GeologyOutline• Stratigraphic contacts– Unconformities• Primary Sedimentary Structures– Younging direction• Intrusive contacts– Sills and dikes– Batholiths• Fault Contacts– Normal, thrust, strike-slip• Folds– Anticlines, synclines1. Superposition: younger layers on topRed layers deposited over tanTan sediment deposited over older rockThird layer is youngest and is on topYoungOldStratigraphic SuccessionBasalt Flows- Snake River Plain2Horizontality of Sedimentary SuccessionsNorth Slope, AKBasic Question:Is the section conformable?• Is the sedimentary pile continuous?UnconformitiesDisconformityAngular UnconformityAngular unconformityDeath Valley3The Great UnconformityGrand Canyon, AZFossils & disconformitiesCorrelation = determine equivalent unitsIHGFEDCBAIHCBAAngular Unconformity4North Slope Basin, AKDownlap surfaceIs the sedimentary succession right-side up?• How can we tell?Which is older?A?B?ABIs the sedimentary succession right-side up?• How can we tell?Primary StructuresPillow BasaltsUp5Sedimentary StructuresStromatolites, Belt Gp., MTUpCross bedded sandstonesUpGraded bedsUpSedimentary StructuresAncient Mudcracks6Mud cracksUPRipplesIntrusive Contacts•Dikes• Sills•Stocks• Batholiths2.CrossCross--cutting relationshipcutting relationshipYounger feature cuts through older rocks or featuresDark igneous rockTan dikesOLDYoung7Coarse-GrainedFine-GrainedIntrusive StructuresDikesSillsStock or batholithSierra Nevada BatholithGeologic history of an areaBased on contact relationships8910Cross Cutting RelationshipsAmato photo, Burro Mts, NM1.6 Ga Amphibolite1.4 GaDikesOrientation of a Plane11Salisbury Crags, Edinburgh, UK, BGS photo Jointed gabbro sillStrike and dip on a mapNorth34382830Folds• Syncline – (concave up)• Anticline– (concave down)12Domes and BasinsTrend and Plunge• Used for linear features• Trend = Azimuth of a line• Plunge = Inclination of a linePlunging FoldTrendPlunging Folds13Does the arrow point at a syncline or anticline?A. SynclineB. AnticlineC. NeitherSheep Mountain Anticline, WY Fault Contacts•Normal• Thrust, reverse• Strike-slip14Thrust FaultFaults in Map View15Key concepts• Stratigraphic contacts– Unconformities• Primary Sedimentary Structures– Younging direction• Intrusive contacts– Sills and dikes– Batholiths• Interpreting sequence on events• Folds– Anticlines, synclines• Fault Contacts– Normal, thrust,