Barrier complexes: Geomorphic and geologic definition Sean Morrison University of Wisconsin-Eau Claire Department of Geology Abstract As sand is deposited along a coastline a variety of landforms are created. Collectively these landforms are known as barrier complexes. Barriers often protect coastal human settlements as well as host hydrocarbon reserves once lithified. An overview of geographic setting, depositional processes and sediment characteristics is therefore provided. Barriers develop in areas of abundant sediment supply and minimal wave and tidal energy. Waves deposit sediment onshore while longshore drift transports sediment along a coast. Depositional environments are created as wave break on a beach and include lower and upper shoreface and beach-dune. Back barriers, washover fans and tidal inlets are also present. Barriers primarily consist of quartz, though other minerals may be present depending on tectonic conditions. To further help understand barrier complexes of the contemporary Columbia River Littoral Cell is described. Generally sedimentogical characteristics are preserved in lithified barriers. To understand how barriers appear in the rock record the Cliff House Sandstone and Pictured Cliff Sandstone of the San Juan Basin are described. Morrison1 Introduction As littoral (near shore) processes redistribute sediments, a variety of landforms are deposited. Siliclastic sediment is sorted and deposited as wave, tidal and wind processes interact to form shore parallel linear barriers. These landforms are typically separated from the mainland by a series of tidal flats, shallow bays, lagoons or marshes, collectively called the back barrier (Davis and Fitzgerald, 2004). Barriers can be differentiated into barrier islands, spits (elongated peninsulas), tombolos (spits connected two land pieces), and cuspates (triangular depositions prograding in either direction) based on their spatial morphology (Figure 1). These configurations show similar lateral and vertical facies. Barriers also are important for human development. Many of the world’s greatest harbors provide safe anchorage for ships because they are protected by barrier complexes. In addition tourists often find the wide expanses of beach sands on barrier complexes (Figure 1). Though spatial extent is important to coastal engineers, barrier facies are easiest to identify in a stratigraphic sequence. This paper will give a broad overview of the processes that create barriers as well as provide facies character of active and sandstone barriers. Figure 1: Aerial photographs with barrier landforms highlighted including, lagoons, barrier islands, spits, tombolos, cuspate forelands. Barriers also serve to protect many coastal human settlements from marine processes. Morrison2 Geographic Extent Barriers form in littoral environments where marine energy and coastal morphology are conductive to barrier deposition. Barrier complexes are found worldwide (Figure 2), they preferentially develop where wave and tidal energy are low (Stutz Figure 2: Barriers are found along the coastline of every continent except Antarctica. Most barriers are found in low energy environments; however, barriers develop in any environment with abundant sediment supplies. (From Stutz and Pilkey, 2011) and Pilkey, 2011). However, where sediment supply is abundant enough to outpace erosive wave and tidal energy barriers develop (Boggs, 2006). Marine processes are the primary drivers in barrier deposition though windblown processes play a minor role in the Figure 3: Map showing tidal amplitude. High tidal amplitude tends to create and erosive environment. Barriers are most common in low tidal amplitude environments. (http://upload.wikimedia.org/wikipedia/commons/5/5e/M2_tidal_constituent.jpg) subaerially exposed portions of the barrier (Davis and Fitzgerald, 2004). Depositional processes are largely controlled by marine and tectonic environments. Marine controls The erosive power of marine processes are largely a function of wave and tidal energy. High wave energy usually results in an erosive environment where most sediment is transported Morrison3 and deposited offshore. Low wave energy results in the deposition of sand grains in the littoral environment and barrier development (Davis and Fitzgerald, 2004). In addition to waves, tides also play an important role in barrier development. Tides are daily rises and falls in sea levels caused by the gravitational effect of the moon and sun on the rotating earth. Tidal amplitude is a result of coastline morphology and deep ocean tidal patterns. A high tidal range produces an erosive environment where sediment is transported offshore and is not conducive to barrier development. Well developed, near continuous barriers are primarily found where tidal range is small to moderate (Boggs, 2006) (Figure 3). Tectonic Setting Davis and Fitzgerald (2004) provide a detailed description of tectonic controls which largely govern sediment supply and to a lesser degree tidal and marine energy. Tectonic setting is Figure 4: Barriers are most common along wide continental shelves (cyan) where tidal and wave energy are low. Subduction zones have narrow continental shelves and are not conductive to barrier development. (http://upload.wikimedia.org/wikipedia/commons/9/93/Elevation.jpg) reflected in an area’s topography (Figure 4). Barriers preferentially develop along passive continental margins. In these settings low relief coastal plains and continental shelves provide an ideal environment for barrier deposition as low relief limits tidal energy. In addition abundant sediment is supplied by extensive continental drainage. Marginal seas show little barrier development. Though plentiful sediment is supplied from active uplift, irregular topography leads to infilling of submarine valleys rather than barrier development. Collision coasts show some barrier development, but only in areas of abundant sediment supply. Sediment is often limited by small river drainage area, in addition, narrow, steep continental shelves are conducive to high wave and tidal energy environments leading to rapid sediment dispersal. Morrison4 Geomorphic Character Before the geologic character can be discussed the geomorphic character must be understood. Uniformitarnism is one of the fundamental principles of geology and states the same natural laws that work on the earth today have always operated on the earth. The fundamental tenet of this theory is that ancient sedimentary sequences were deposited in the same fashion as modern sedimentary sequences. A firm understanding of depositional processes and facies sequences of active barriers is therefore essential when studying sandstone barriers. Sediment Deposition and Barrier Migration Longshore drift (process of transportation of sediment along a coast) dictates the direction of sediment movement and barrier migration along a coast. The direction of longshore drift is dictated by wind direction. Waves are pushed on shore by wind energy, usually at an angle. Sediment is pushed Figure 5: Sediment transport along a coast dominate by longshore drift and wave action. As wave approach shore sediment is moved onshore then returns offshore. In this manner sediment is moved in a zig-zag manner in the direction of longshore drift. (http://homertribune.com/wordpress/wp-content/uploads/2010/06/drift.gif) up a beach with the wave then pulled back oceanward. Since the wave refracts perpendicular to the shoreline sediment is pulled oceanward. Another wave then remobilizes the sediment particles, pushing the sediment toward shore in the direction of the oncoming wave. Repetition of this process moves sediment in along a coast in a zig-zag fashion and can cause a barrier to migrate down current. (Figure 5) (Davis and Fitzgerald, 2004). Depositional Environments Barriers develop in response to changing sediment supply and sea level fluctuations. Depositional processes vary in each area of a barrier system and are largely influenced by wave action. The orbital pattern of wave motion compresses as the waves approach shore creating several zones (Figure 6). The wave base is defined as depth at which a wave’s passage causes significant water motion. The area above the mean fair weather wave base is termed the Morrison5 shoaling zone and is further subdivided into the surf and breaker zones. As breaking waves generate turbulence they throw up suspension sediment clouds as well as cause a net landward migration of bedload sediment. The area of Figure 6: Various zones are defined as a wave compresses and breaks as it approaches shore. Beginning at the mean fairweather wave base a wave first enters the breaker zone followed by the surf and swash zones. (From Boggs 2006) sediment movement is the called the surf zone. Furthest from shore, the breaker zone begins with the wave base and ends at the surf zone. This area is dominated by oscillatory wave motion (Davis and Fitzgerald, 2004). Between the low and high tide line, the swash zone is dominated by sediment laden swash up the beach and backwash off the beach. By using wave motion and relation to the barrier several depositional environments can be defined (Figure 7). a) Marsh-lagoonal – Marsh-lagoonal are deposited landward of the barrier and vary depending on the exposure of the back barrier to the open ocean. Partial inundation of a back barrier may result in an intertidal flat or salt marsh while total inundation of a back barrier forms a lagoon or tidal creek (Davis and Fitzgerald 2004). b) Washover Fans – When storm tides overtop beach-dune sediments is often eroded and redeposit on top of marsh-lagoonal sediment. Channels facilitate the transport of large amounts of sandy sediment into the back barrier. Repeated washover events coalesce washover fans into large complex deposits (McCubbin, 1982). c) Tidal Inlet channels – Inlet channels connect back barrier environments to the ocean and range in depth from 4.5m to 40m. The size and spacing of inlet channels is dependent on wave and tidal regime. Reversing tidal flow, waves and currents interact to deposit and erode material in inlet channels. In deep channels seaward current dominates while shallow channels are dominated by landward current. The ebb (oceanward) and flood (landward) tidal deltas form at the outlet of tidal inlets based on dominate flow characteristics (McCubbin, 1982). Morrison6 d) Beach-dune – Wave swash and windblown processes dominate beach-dune deposition. Tides shift swash laterally and vertically on the beach while wind may form dunes, depending on vegetation cover, above the high tide line (McCubbin, 1982). e) Upper shoreface – The surf zone and upper breaker zone constitutes the upper shoreface, depth varies and is a function of wave amplitude (Davis and Fitzgerald, 2004). This zone is dominated by Figure 7: Depositional zones in a barrier system. Depositional zone are primarily dictated by wave patterns the protection provided by barrier system. Zones include the back barrier, washover fans, tidal inlet channels, beach-dune, upper shoreface and lower shoreface. (From McCubbin 1982) oscillatory wave motion (McCubbin, 1982). f) Lower shoreface – The lower breaker zone to the fair weather wave base is called the lower shoreface which marks the transition between littoral and off shore processes. This zone is dominated by low energy suspension settling (Davis and Fitzgerald, 2004). Facies sequences A complex set of facies are observed within barriers and reflect depositional processes. Predominate facies include marsh-lagoonal, washover fan, tidal inlet channels, beach-dune, upper and lower shoreface sediments (Figure 7 and Figure 8). The primary mineral in most barriers is quartz since marine currents and wave action sort out most fine grain and liable sediments (McCubbin, 1982). Facies thickness varies in high and low energy environments. Morrison7 Generally upper and lower shoreface facies are thinner in low energy environments than high energy environments where erosive processes move sediment from the beach and deposit it on the upper shoreface (McCubbin, 1982). a) Marsh-Lagoonal – Horizontal to subhorizontal interfingered sand, silt, mud and peat that laterally grade to high energy sands near tidal channels, deltas and washover fans. Commonly bioturbated with brackish water invertebrates (eg oysters) and plant remains (Boggs, 2006). b) Washover Fans – Horizontal to subhorizontal, may terminate with foreset sands, decreasing thickness away from apex, bioturbation in upper layer, oceanic debris also present. Indvidual deposits can be 1m thick and extend landward up to 300m, however, amalgamations of several washover fans may create large, complex deposits (McCubbin, 1982). c) Tidal inlet – Thickness of sediment is dependent of channel depth and may locally replace all other beach facies. Generally, deposits consist of medium to coarse grain and show a fining upward sequence. Tabular, planar stratification dominates the channel bottom. Spit migration follows longshore drift and show trough shaped cross-stratification as well as mega-ripple and large scale foresets. Mixed ocean and marsh-lagoonal fauna may be present with bioturbation. Ebb and flood tidal deltas are characterized by ebb or flood oriented planar cross stratification, depending on delta type and dominate flow with small scale bidirectional, tough cross Figure 8: Idealized cross-section of beach-dune, upper shoreface and lower shoreface grading to offshore and continental shelf sediments in an active barrier. (Modified from Boggs 2006) stratification (McCubbin, 1982). d) Beach-dune – Planar to seaward dipping (2̊-10̊) laminations, fine to medium sands, root traces. Wedge shaped set commonly occur due to varying wave conditions (McCubbin, Morrison8 1982). Windblown sands may be cross-stratified. Swash concentrates relatively coarse particles as well as heavy minerals in thin laminations (Boggs, 2006). e) Upper shoreface – Crossbedded laminations, Medium grading to fine sands, minor bioturbation (Boggs, 2006). Trough bounded cross-stratification common and are produced by the interaction of littoral currents and longshore drift (McCubbin 1982). f) Lower shoreface –Fine sands grading to silts, extensive bioturbation (Boggs, 2006). Ripples grading to horizontal to subhorizontal stratification. Individual beds may show hummocky cross-stratification (McCubbin, 1982). Modern example Vanderburgh et al. (2010) and Peterson et al. (2010) sampled barriers within the Columbia River Littoral Cell (CRLC) inferring much about its formation from the stratigraphy of several barriers. Drill cores were primarily used to sample the barriers internal stratigraphy. In addition ground penetrating radar was used to image the radar facies of the shallow subsurface. The CRLC is located near the mouth of the Columbia River along the western margin of the Figure 9: The Columbia River Littoral Cell (CRLC), near the mouth of the Columbia River along the western margin of North America shows many well developed barriers. (From Peterson) Figure 10: Several drill cores taken from various subcells within the CRLC show the stratigraphy of barriers. (From Peterson) Morrison9 North American Plate (Figure 9). Sediment is supplied by the Columbia River and reworked by wave, tidal and windblown processes. High wave energy from a thin continental shelf and Pacific Ocean storms creates a broad zone of offshore sand mobilization while northward longshore drift is generally responsible for sediment transport. Stratigraphic sections for CRLC barriers are based on several drill cores (Figure 10) and show typical barrier facies. Barrier facies begin with a laterally extensive, low gradient ravinment surface that cuts underlying strata presumable from the transgression and erosion of lowstand topography. In parts, this erosion deposited a thin, 0.25m-1.83m, transgressive lag of rounded to subrounded gravels in a sand matrix with shell fragments, detrital wood and opaque minerals. Lags often fine upward to sands. The dominant unit in CRLC barriers are shoreface deposits, lower and upper shoreface deposits were not differentiated. These deposits range in thickness from 8.0m on North Beaches to 26.0m on Clatsop Plains and consist of well sorted, weakly layered dark gray fine to medium sands (with a man grain size of 0.20mm) with rare mud-drape ripples. Scattered pebbles range in size from 4 to 10mm with infrequent wood and shell fragments. Radar stratigraphic interpretation of ground penetrating radar (GPR) differentiated beach-dune from shoreface deposits. A 1.7m on North Beaches to 15.5m on Long Beach thick radar facies of seaward dipping clinoforms were interpreted as beach deposits. The thickest barrier deposits were observed on Long Beach and Clatsop Plains (Figure 10), the barriers immediately adjacent to the Columbia River outlet and the main source of sediment input (Figure 9). This emphasizes the importance to sediment supply in barrier development. Even with high tidal energy (Figure 3) and wave energy from a narrow continental shelf (Figure 4) a series of well-developed barriers has developed thanks to abundant sediment supplied by the Columbia River with the largest barriers developing immediately adjacent to the Columbia River outlet. . Geologic Character The importance of the lithified coastal systems to human development cannot be under stressed. Shifting shorelines often yield hydrocarbon producing units from stacked transgressive and regressive barrier sequences. Ebb and flood-tide deltas associated with tidal inlets, overwash fans and a shift in shoreface deposits often trap organic rich marsh-lagoonal source rock between highly permeable sandstone units. Further tilting or anticline formation can then create a variety Morrison10 of traps (Morse, 1994). The geomorphic character of barrier facies are generally preserved during lithification including mineralogical and bedding characteristics (Flores and Erpenbeck, 1981). However transgressions (relative sea level rise) and regressions (relative sea level fall) often complicate the lateral and vertical facies in barrier sandstones. During transgressions barriers upper shoreface and beach-dune deposits are eroded as a new barrier develops along the new coastline. A new Figure 12: The San Juan Basin in north-west New Mexico located in Northwest New Mexico provides examples of several lithified coastal systems. (http://en.wikipedia.org/wiki/File:SanJuanBasinUSGS.jpg) facies sequence generally develops ontop of previously deposited marsh-lagoonal sediments. Regressions often cause barriers to prograde seaward as sea level falls causing strandplain expansion and preserving facies sequences as a barrier builds outward (McCubbin, 1982). The San Juan Basin in northwest New Mexico (Figure 12) provides several examples of lithified barrier systems. Units were deposited in the foreland basin of the Figure 13: Cross-section of San Juan Basin. Units were deposited in the Cretaceous Inner Seaway. The Cliff House Sandstone was deposited during a series of transgressive and regressive cycles. The Pictured Cliff Sandstone was deposited during the final regression of the Cretaceous Inner Seaway. (From Donselaar, 1988) Cordilleran uplift along the shore of the Cretaceous Inner Seaway that connected the Gulf of Mexico to the Arctic Ocean. The Upper Cretaceous Pictured Cliff and Cliff House Sandstone (Figure 13) are believed to have been deposited during the final retreat of the Cretaceous Inner Seaway during several transgressive and regressive cycles. Donselaar (1988) and Flores and Erpenbeck (1981) describe lithic barrier sequences within the San Juan Basin. The units described are the Cliff House and Pictured Cliff Sandstones respectively. Cliff House and Pictured Cliffs Sandstone grades into the lower, non-marine Menefee and Fruitland Formations. In the majority of the San Juan Basin the Pictured Cliff and Cliff House Sandstones are separated Morrison11 by the offshore marine Lewis Shale but the sandstone units merge where the Lewis Shale pinches out. Cliff House Sandstone Four stacked sandstone units in the Cliff House Sandstone identified as barrier complexes (Figure 14) and described by Donselaar (1988). Outcrops are exposed along Chaco Canyon, New Mexico (Figure 15). The Cliff House Sandstone is a 90m thick, 9km wide, linear sandstone complex composed of very fine grained to fine grained lithic arenite subdivided into four stacked sandstone bodies. Marsh-lagoonal deposits are identified as Menfee Formation shale. Very fine grained, cross bedded tidal inlet deposits are lenticular sandstone bodies with a slightly scoured lower surface. Cross beds occur as 10-30 cm high sets. Washover fans occur as of horizontally laminated sandstone sandstone units up to 30cm thick. Upper shoreface deposits are dominated by parallel and cross-bedded very thickly bedded sandstones. Lower shoreface deposits occur as bioturbated, cross-laminated, thickly to thinly bedded, horizontally bedded sandstone and grade to offshore Lewis Shale deposits. Transgressions are marked by a sharp erosional surface through marsh-lagoonal deposits Menefee Formation shale. Stacking of four barrier sandstone units (Figures 14-15) are believed to represent a series of transgressions and regesions along the Cretaceous Inner Seaway. A model for the stacking of transgressive and regressive units in proposed from interpretation and analysis of sandstone bodies. While during regression sediment is deposited and a coarsening upward barrier facies sequence develops. Transgression are marked by a sharp Figure 14: Inferred depositional environment of stacked sandstone bodies within the Cliff House Sandstone and associated Lewis Shale (Kl), Menefee Formation (Kmf) and La Ventana Tongue of the Cliff House Sandstone (Kchlv). (From Donselaar, 1988) Figure 15: Modern setting of Cliff House Sandstone across several transects of Chaco Canyon, NM. Stacked sandstone bodies 1-4 correspond to sandstone bodies in Figure 13 (From Donselaar, 1988). Morrison12 erosional contact. Upper deposits are often eroded away, preserving shoreface and marshlagoonal deposits. Four sandstone bodies are differentiated by the identification of sharp erosional contact surfaces and the deposition of lower shoreface sediment on top of the previously deposited sediment. Cliff House sandstone stratigraphy shore four stacked barrier sequences (Figure 17) representing four regressive sequences during relative sea-level rise during the Cretaceous. Figure 17: Stratigraphic columns of several section of the Cliff House Sandstone exposed near Chaco Canyon. Morrison13 Pictured Cliff Sandstone Barrier lithofacies within the Pictured Cliff Sandstone are described by Flores and Erpenbeck (1981) and represent the final regression of the Cretaceous Inner Seaway. In general a coarsening upward sequence from very fine to medium grain quartz. Mineral composition averages 43% (in upper units) to 74% (in lower units) quartz, 14% feldspar, 4% mica fragments and 20% lithics. Lower shoreface deposits consist of interbedded sandstones, siltstones and shales ranging in thickness from a few millimeters to 1.5m and grade downward into Lewis Shale which likely represents offshore deposition. In general, lower shoreface sediments are dark gray, laminated and show ripple lamininations where not bioturbated by skolithos or Figure 18: Cross-sections of barriers within the Pictured Cliff Sandstone (From Flores and Erpenbeck, 1981) ophiomopha burrows. Grain size ranges from silt to fine grain. The upper shoreface is light gray where unweathered with low angle crossbeds and locally bioturbated and ranges in thickness form 6m to 9.5m. Grain size ranges from fine to medium grain. Beach-dune deposits are light gray showing parallel to subparallel laminations that locally grade to crossbeds and ripple laminations with intense sparsely to locally ophiomorpha burrows. Deposits range in thickness from 4.5m to 6m. Grain size ranges from fine grain in the lower part of the unit to medium grain in the upper part of the unit. Rare washover fan deposits occur in the overlying back-barrier Fruitland Formation. Tidal inlet deposits occur as 2.5km wide elongate deposits with a thickness of 11m. Lenticular and foreset laminations with some ripple laminations and ophiomorpha burrows occur within the deposits. Lower units contains fine grains with shale and siltstone lag clasts grading to medium grains (Figure 18). The size of coal beds in the overlying Fruitland Formation is determined by their association with barrier and delta-plain deposition in the Pictured Cliff Sandstone. Thin coal Morrison14 beds, 0.6m thick, in the lower Fruitland Formation represent back barrier deposition. Thicker, economic coal beds are found in delta-plain deposits. The identification of delta-plain versus barrier deposits in the Pictured Cliff Sandstone can therefore be used to determine the nature of overlying coal deposits. Conclusion Hydrocarbon reservoirs and the protection barriers offer to coastal human settlements make barriers one of the most important depositional environments on the earth’s surface. Siliclastic barriers develop from the interaction of wave and tidal, and to a lesser extent windblown, processes. Barriers most often occur where wave and tidal energy is low and sediment input is high. As waves break against the shoreline they create a variety of depositional environments including, lower and upper shoreface and beach-dune. In addition marsh-lagoonal, washover fans and tidal inlets are defined by their association with the main barrier. The Columbia River Littoral Cell provides an example of a modern depositional environment and emphasizes the importance of sediment input to barrier development. The Cliff House and Pictured Cliff Sandstone provides an example of an ancient barrier complex and the show the complications in barrier facies that arise from regressive and transgressive phases and the importance of barrier complexes in hydrocarbon reserves. References Boggs, S., 2006, Principles of Sedimentology and Stratigraphy, 5th ed., Pearson Prentice Hall, p. 260-267. Davis, R. A. and Fitzgerald, D. M., 2004, Beaches and Coasts: Massachusetts, Blackwell Science Ltd., p. 101-167. Donselaar, M. E., 1988, The Cliff House Sandstone, San Juan Basin, New Mexico; model for the stacking of "transgressive" barrier complexes: Journal of Sedimentary Petrology, v. 59, no. 1, p. 13-27. Flores, R. M., and Erpenbeck, M. F., 1981, Differentiation of delta-front and barrier lithofacies of the Upper Cretaceous Pictured Cliffs Sandstone, southwest San Juan Basin, New Mexico: The Mountain Geologist, v. 18, no. 2, p. 23-34. McCubbin, D., 1982, Barrier-Island and Strand-Plain Facies: Sandstone Depositional Environments: AAPG Memoir 31, p. 247-280. Morse, D.G., 1994, Siliclastic Reservoir Rocks: The petroleum system – from source to trap:AAPG Memoir 60, p. 121- 140. Peterson, C. D., Vanderburgh, S., Roberts, M. C., Jol, H. M., Phipps, J., and Twichell, D. C., 2010, Composition, age, and depositional rates of shoreface deposits under barriers and beach plains of the Columbia River littoral cell, USA: Marine Geology, v. 273, no. 1-4, p. 62-82. Stutz, M. L. and Pilkey, O.H., 2011, Open-Ocean Barrier-Islands: Global Influence of Climatic, Oceanographic, and Depositional Settings: Journal of Coastal Research vol. 27, iss. 2, pp. 207-222. Vanderburgh, S., Roberts, M. C., Peterson, C. D., Phipps, J. B., and Herb, A., 2010, Transgressive and regressive deposits forming the barriers and beachplains of the Columbia River Littoral Cell, USA: Marine Geology, v. 273, no. 1-4, p. 32-43.