Geology of the San Juan-Cascade Nappes, Northwestern Cascade Range and San Juan Islands

Geology of the San Juan-Cascade Nappes, Northwestern Cascade Range and San Juan Islands Mark T. Brandon Department of Geology and Geophysics Yale University P.O. Box 6666 New Haven, CT 06511 INTRODUCTION The San Juan-Cascade nappes (Fig. 1) lie in a on the following three topics: {l) pre-Late Cretaceous ter northwest-trending belt which is bounded by the Skagit ranes within the nappe sequence, (2) continental accre metamorphic core (Haugerud, this volume) to the north tion of these terranes prior to the Late Jurassic, and (3) the east and Wrangellia to the southwest. Wrangellia is a tectonic setting associated with Late Cretaceous thrust large coherent terrane that underlies much of Vancouver ing. Island. At present, the nappe sequence has a cross-strike width of at least 150 km, extending from the westernmost DISCUSSION thrust in the San Juan Islands (labeled Late Cretaceous suture in Fig. 1) to the Ingalls ophiolite in the Mount Pre-Late Cretaceous Terranes in the Stuart area (shown as Decatur terrane on Fig. 1). A com San Juan-Cascade Nappes mon interpretation is that the San Juan-Cascade nappes represent a long-lived accretionary system that formedby The San Juan-Cascade nappes are commonly iden successive arrival of far-traveled terranes (for example, tified as a composite terrane (for example, Jones and Whetten and others, 1978). This view is certainly consis others, 1983) because of the pervasive effects of Late tent with the varied assortment of Mesozoic and Cretaceous thrusting. In the extreme, each nappe or fault Paleozoic terranes in the nappes. More recent work slice might be considered a separate terrane,and the Late (Brandon and others, 1988), however, has shown that Cretaceous faults viewed as fundamental terrane boun large parts of the nappe sequence were assembled during daries. A more conventional approach is to use geologic a short-lived orogenic event, bracketed between100 and relations within each nappe to piece together the laiger 84 Ma. Brandon and Cowan (1985) argue that this tectonostratigraphic units that existed prior to the Late orogenic event was associatedwith the collision of Wran Cretaceous. An example of this approach is the analysis gellia. The San Juan-Cascade nappesare considered to be of Brandon and others (1988), which employs a system of Late Cretaceous thrust sheets that were driven stratigraphic relations, paleontological ages, isotopic to the southwest over Wrangellia. Many parts of the dates, and chemical analyses to identify and characterize nappe sequence were also affected by a synchronous pre-Late Cretaceous terranes in the San Juan Islands. The high-pressure metamorphic event (characteristic as results of this analysis areextended here to all partsof the semblage: prehnite-lawsonite-aragonite) caused by rapid San Juan-Cascade nappes (Table 1, Figs. 1 and 2). This structural burial and subsequent uplift within an advanc analysis should beviewed as preliminary considering that ing thrust wedge (Brandon and others, 1988). the internal stratigraphy and age of many of the Cascade The discussion section of this paper provides an nappesare just starting to be resolved. overview for fieldstops in the San Juan-Cascade nappes, Seven pre-Late Cretaceoustectonostratigraphic units which are Stops 2-7 to 2-11 in the western Cascades are identifiedin Table 1 (shown as major headings). Each (Haugerud, this volume) and Stops 3-1 to 3-8 in the San of these contains one or more affiliated rock units (listed Juan Islands (described below). The discussion focuses below the major headings). Three types of tee- From:Joseph, N. L, andothers, editors, 1989, Geologic guidebookfor Washington and adjacent areas: Washington Divisionof Geology and EanhResources Information Circular 86. 138 INFORMATIONCIRCULAR 86 ' ' EASroN TERRANE 11!111 ·� V>� � � DECAM TERRANE �o· · CONSTITUTION FM. · . a OLOMT UNIT <f� DEADMAN BAY TERRA� cH1wwAcK TERRANE ��Lm WRANGELLIATERRANE ml 0 20 40 60 80 IOO KM - SKAGIT '-ETAMORPHIC I I I I I I ll"tlll CORE Figure 1. Generalizedmap of tectonostratigraphic unitsin theSan Juan-Cascade nappes.The Straight Creek fault is re storedbased on the interpretationof Misch (1977b), which matches metamorphic units in the Skagit Core (Chiwaukum andSettler Schists). TheVedder terr ane, Richardsonbasalts, andLopez Structural Complex are toosmall to show here. One exceptionis alarge outcrop of Vedderterr aneat VedderMountain (labeled"Vedder Complex"). The east-westline across the SanJuan Islands andMount Baker locates the cross-sectionin Figure 4. Thefault labeled" LateCretaceous suture"marks the westernlimit of the SanJuan-Cascade nappes. The fault labeled"Cenozoic truncation scar" marks the location of aninferred early Cenozoic fault thattruncated the southwest sideof the Cascadeorogen (Brandon,1985). LATE "' ::::> @ � � EARLY NOOKSACK GRP LATE WELLS CK 8 1,1 HARRISON LK MIDDLE VOLCS � EARLY � ORCAS C.ULTIJS 8 2 5 GARRISON 8 LATE CAMP COVE @ CHERT LATE � VEDOOl FM S � tl:1 TRAF� SClfSTS � MID8EARLY � � Mil8 EARLY UNITS (META AGE) ELOON LK FM. ocm PERMAN @r----1 BAY VOLC. � PENNSYLVAMAN CH ILLIWACK LATE 8 AllLEOZOIC MISSISSIPPIAN GRP i SICKER GRP L:JDCD � DEVONIAN btrl 0 PRE TURTLEBACK PRE- DEVONIAN t'"' DEVONIAN ex 8-< TERRANE NAME> WRANGELUA i..1 ...- ---?-CHILLIWACK:----l.,...,l ,..____,DEADMAN BAY-----.j VEOOER l"'"•---- DECATUR�------..MI EASTON � Figure 2. Pre-Late Cretaceous terranes in the San Juan-Cascade nappes (modified from Brandon andothers, 1988; see Table 1 forfurther en� details). Each columnrepresents a separate terranewith its nameappe aring below.The continuousportion of a column shows rock units that areconnected by demonstrablestratigraphic contacts.Breaks in a column indicate that stratigraphic contactsare not preserved. Stratigraphic � hiatuses arehachured, anddashed horizontal lines indicate that the age range of theunit is not known. Arrows with numbers indicate other � types of geologic ties between rockunits within a terrane.The numbers refer to the followingcomments: (1) dikes relatedto the EastSound �I Group are found in the Turtleback Complex, anda poorly preserved unconformity between the two units is locally present (Brandon and (') others, 1988); (2) cobbles of Chilliwack Group limestone arefound in the Camp Cove Formation (Monger, 1986; Monger andBerg, 1987); > en (3) similarities in chert ages andbasalt chemistry (Brandon and others, 1988); (4) Tethyan fusulinid-bearing limestone cobbles in the Con (') stitution Formation (Brandon and others, 1988); and (5) Garrison schist cobbles and detritus in theConstitution Formation (Brandon and > others, 1988). Stippled units are Jurassic andCretaceous elastic linking sequences,which are inferred to postdate accretion of the pre-Late � Cretaceous terranes to continentalAmerica. The timing of Late Cretaceousthrusting andregional high-pressure metamorphism is shownat z the top of the diagram.Heavy linesare meant to schematically representthrust faults within the SanJuan-Cascade nappes. The lower flats of these faults underlie theunits that they involve, and the upper flats marks the time of movement of the fault The Nanaimo Group is a syn trl orogenicunit thatwas depositedon Wrangellia andreceived detritusfrom the SanJuan-Cascade nappe s. en� .... w '° 140 INFORMATIONCIRCULAR 86 Table 1. Pre-Late Cretaceousunits inthe SanJuan-Cascade nappes. Tectonostratigraphicheadings arelisted in an order that approximatestheir relativepositions, fromtop tobottom, in the nappesequence. ================================================================================================ BASTON TERRANB <METAMORPHIC) Baston Metamorphic Suite (= Shuksan Suite of Misch, 1966) [VC & CC; 2, 12, 14): Metamorphism is primarily Early Cretaceous (120-130 Ma). A coherent high-pressure metamorphic unit composed of basalt (ocean-floor chemistry), graphitic quartzose phyllite, minor sandstone, and rare metalliferous sediments. Locally includes minor Late Jurassic barroisite schist. Isotopic dating suggests a Jurassic depositional age for the elastic sediments. DECATUR AND RELATED TERRANES (COHERENT> Middle and Late Jurassic ophiolite and arc-volcanic complex; also includes overlying Jura Cretaceous elastic strata. Fidalgo Igneous Co•plex [SJ; 1): Middle and Late Jurassic. Ultramafite, gabbro, and low potassium basalt, andesite and dacite. In the San Juan Islands, the associated volcanic sequence contains interbedded radiolarian argillite and chert, and breccia with clasts derived from the underlying plutonic and volcanic sequence. Miscellaneous ophiolitic units in the Cascades Foothills [CF; 9, 11): Middle and Late Jurassic. A group of ophiolitic units which based on age and lithology are broadly correlative to the Fidalgo Complex. Includes the Twin Sisters dunite, the Haystack unit (9, 10], the Stillaguamish ophiolite [ll], volcanic and plutonic rocks of the Vestern Melange [8], and the Deer Creek volcanics [2]. Ingalls Co•plex [CC; 3,4): Late Jurassic. A complexly faulted assemblage of ultramafite, gabbro, mafic and silicic volcanics, ribbon chert, and sandstone. Lummi Group [SJ; 1, 13): Latest Jurassic and Early Cretaceous. Marine elastic strata overlying the Fidalgo Complex. elastic units associated with ophiolitic rocks of the Cascades Foothills (CF): Late Jurassic and Early Cretaceous. Consists of poorly exposed and variably disrupted mudstone, turbidite, conglomerate and minor chert. Includes the Sultan unit (7) and units pTwa and pTwak in the Vestern Melange (8). PILLOV BASALTS OF RICHARDSON <EXOTIC FAULT SLICE) Basalts of Richardson

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