sedimentology

Fan Deltas
All images and text copyright Peter M. LeTourneau, unless otherwise attached to publication authority. Images and text may not be used without specific permission of the author.

[Draft, 2007]

Early Jurassic rift basin fan delta, Hales Brook Fan, Hartford Basin, Portland Connecticut

[Portions of the following text originally published in: Olsen, P.E., Whiteside, J., LeTourneau, P.M., Huber, P. (2005) Jurassic cyclostratigraphy and paleontology of the Hartford basin. In McHone, N.W. and Peterson, M.J. (eds.) Guidebook for fieldtrips in Connecticut - New England Intercollegiate Geologic Conference, Yale University, New Haven. State Geological and Natural History Survey of Connecticut, Guidebook No. 8 Trip A-4, pp. 55-106]

Peter M. LeTourneau

Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964-8000

"... on the very margin of the valley, we find a coarse conglomerate in a few places, of quite peculiar character...The fragments are sometimes several feet in diameter, and the stratification of the rock is very obscure."
Edward Hitchcock, Ichnology of New England, 1858

"In the Connecticut Valley a coarse conglomerate, sometimes containing rounded bowlders (sic) two or three feet in diameter, occurs along the eastern margin of the area and about its northern end, but is seldom seen on its west border."
Israel C. Russell, Correlation Papers: the Newark Group, 1892

                    Fig. 1. Sedimentary features of the Hales Brook fan-delta complex, South Glastonbury, Conn.

Introduction

In 1991 an extraordinarily illustrative exposure of very coarse alluvial fan conglomerate and finely laminated, fossiliferous black shale was uncovered in a sand and gravel quarry in Portland, Connecticut. This exposure is an unsurpassed example of fan-delta deposition at the faulted margin of the Hartford Basin. The purpose of this paper is to describe this remarkable fan delta sequence within the context of lower Jurassic depositional environments of the Portland Formation in central Connecticut.

In the context of previous studies of the alluvial fan deposits of the Portland Formation, this new occurrence is considered the type fan delta facies association in the Hartford Basin for the following reasons: 1) accessibility and lack of vegetative or soil cover; 2) great range of grain size, from clay to boulders over 2 m in length in close stratigraphic proximity; 3) evidence of rapid sub-aqueous fan deposition including turbidite beds and extraordinary slump folds; 4) abundant fossils including fish, conchostracans, and plants; 5) dramatic coarsening-up sequence; 6) one of the few well-exposed fan-lake sequences found within 1 km of the eastern fault margin of the basin; and 7) extraordinary three-dimensional exposure showing bedding plane, along-strike and along-dip views of beds and sedimentary structures.

The exposure has great utility for educational purposes for demonstrating diverse geological principles including the following:
    Sedimentology: finely laminated lake shale, shallow water near shore sandstone, sub-aqueous mass flow deposition (turbidites), slump folds and soft-sediment deformation, sub-aqueous and sub-aerial conglomerate; fan progradation, Walthers Law;
    Structure: post-depositional normal faults, effects of bed strength on fault plane attitude, bedding plane shear
    Paleontology: modes of fossil fish preservation, paleoecology, lacustrine environments
    Glacial Geology: glacial striations, effect of bedrock on ice flow; ice contact deposits.

Location and description

The exposure is located at 41o 37" W lat.; 72o 38' N long. near the northwest border of Portland, Connecticut about 4 km south of South Glastonbury center and about 1 km west of Route 17 which is roughly coincident with the eastern fault margin of the Hartford Basin. The site is accessed through private property located south of Old Maids Lane, about 1/2 mile from the intersection of the lane with Rt. 17.

The exposure was uncovered during routine excavation of glacial outwash for sand and gravel. Previous geological reconnaissance revealed the presence of small scattered outcrops of boulder conglomerate at elevations above 200 ft (msl) on the small isolated hill located east of the sand and gravel quarry, but no dark shale beds were previously found in the area (LeTourneau, 1985).

The exposure is about 150 - 200 ft. long and consists of a north-facing vertical section of dark shale, sandstone and conglomerate and a broad, south-facing dip slope of boulder conglomerate. Bedding strikes N 70 W and dips about 25 degrees SW (110-25). The eastern and western ends of the outcrop are terminated by normal faults striking roughly north-south. The entire outcrop shows evidence of glacial scour with deep, sub-parallel grooves and scratches that envelope bedrock surfaces. The glacial striations also wrap around the eastern and western ends of the exposure, providing dramatic evidence that bedrock influenced the local flow path of the base of the overlying ice sheet.

The exposed strata coarsen upward and exhibit an extremely wide range of grain sizes and bedding style, from thin-bedded finely laminated dark shale to crudely stratified boulder conglomerate within the relatively thin, but continuous vertical section. The depositional environments represented include deep water lacustrine, shallow or littoral lacustrine, sub-aqueous fan-delta, and sub-aerial alluvial fan. Clast grain size ranges from mud to very large boulders (over 2 m on longest exposed axis).

Hales Brook Fan Delta
The outcrop described herein is located within 1 km of the eastern fault margin of the basin in an area of very coarse conglomerate previously identified as the Hales Brook Fan (LeTourneau, 1985b). The exposed strata coarsen upward and exhibit an extremely wide range of grain sizes and bedding style, from thin-bedded, finely laminated dark shale to crudely stratified boulder conglomerate within the relatively thin, but continuous vertical section. The depositional environments represented include deep water lacustrine, shallow or littoral lacustrine, fan-delta, and alluvial fan. A measured section ([Fig xx] shows the stratigraphic succession from very fine grained lacustrine beds through exceedingly coarse boulder conglomerate beds.

The fan delta sequence is divided for reference into 6 units [Fig x] , from base to top. Unit I at the base of the section is a poorly exposed pebble and cobble conglomerate more than 4 m thick. The exposed portion of conglomerate is undoubtedly part of a larger conglomerate sequence as suggested by bedrock "float" and the east-west oriented resistant ridge which contains the Unit I rock. A covered interval of approximately 4.5 m overlies Unit I and forms the base of Unit II; 2.2 m of finely laminated, fossiliferous black shale. Unit III is about 4 m thick and consists of three fining-up sub-units of interbedded sandstone and siltstone. Unit IV is a 0.5 m bed of coarse to pebbly sandstone with a sharp basal contact and gradational upper contact. Unit V forms the top of the exposed section and consists of very coarse boulder conglomerate. The sedimentary features and paleoenvironmental interpretation of these units are discussed below.

    Unit I
The base of the outcrop consists of poorly-sorted, poorly-stratitified conglomerate with few thin silty sandstone partings or thin beds. Although the exposure of this bed is poor, a fining-up trend can be observed in the sized of the major conglomerate clasts. Clast composition is polymict and reflects the varied low- to high-grade metamorphic rocks and plutonic igneous rocks. Clasts are sub-rounded to sub-angular; no preferred orientation could be observed. Unit I is interpreted as an alluvial fan deposit based on comparision with other well-known ancient examples including the Hartford Basin (e.g. Steel, Nilsen, 1982, Letourneau, 1985a) and analysis of modern depositional enviroments (e.g. Bull, Nilsen, 1982)

    Unit II
The base of Unit II is a 2 m covered interval. The upper 0.75 m consists of finely laminated, thin bedded black to dark gray, organic-rich, fossiliferous shale. Bedding contacts are sharp and planar and laminations may be traced through the exposed portion of Unit II. The upper portion of Unit II contains a few fining-up siltstone interbeds or lenses. Laminations consist of clastic and carbonate couplets (varves, sensu Olsen, 1986).   Evidence of subaerial exposure such as mud cracks or root burrows are not present nor are invertebrate burrows observed. Fish fossils are whole and articulated, but are in some cases distrupted by normal faults and bedding plane faults of small displacement that penetrate portions of the outcrop.   These features correspond to those described by Olsen (1984, 1986) for deep water lacustrine strata within the Newark Supergroup.   The sedimentary structures and preservation of whole articulated fish are indicative of deposition below wave base and the thermal or chemical stratification of the lake water column. Therefore, Unit II is interpreted as a perennial, stratified lake deposit based on comparison to examples of modern and ancient rift lakes (e.g Olsen, var.; Dean, 1982; Trewin, 1986)

Fossils from Unit II include Seminotiid fishes, conchostracans, and plants. The excellent preservation of the wholly articulated fish is typical of fossil fish found within a lateral distance of 1 to 2 km of the eastern basin margin. McDonald and LeTourneau (1989) attribute geographic trends in fossil fish preservation to relatively high rates of sedimentation adjacent to the basin margin. The relatively rapid burial of fish carcasses prevents microbially mediated dephosphatization of bones (Nriagu, 1983) which is seen to occur in lake beds located in central and western areas of the Hartford Basin. In addition, the size of Unit II conforms the bed thickness predicted by the previously determined relationship of lake bed thickness and distance from the faulted basin margin for the Hartford Basin [Fig. xx].

    Unit III
Unit III consists of 1.3 m of interbedded gray mudstone, thin normal-graded sandstone to siltstone beds, and light gray to brown medium to coarse sandstone with subordinate granule lenses and small pebble layers and lenses. The gray mudstone is thin-bedded with planar horizontal lamination, and minor pinch and swell lamination. The mudstone beds include rusty-weathering ferroan dolomite-rich (e.g. Hubert et al., 1978) beds and nodules. The thin normal-graded sandstone-siltstone beds occur in repetitive bed sets throughout Unit III. Thick slump folds and soft sediment deformation are noteworthy in this Unit.

A large slump fold is observed in the central portion of the exposure. The recumbent fold is about 1 m thick and 1.5 m wide with a rounded lower boundary and a shallow, convex upward upper surface. Laminations of fine and coarse sand, often as normal-graded couplets, within the slump are crenulated, recumbent, and of different attitude than the laminations on the outermost portion of the slump fold, indicating that the internal deformation was somewhat independent of the shear forces that shaped the outer boundaries of the slump fold [Figxx]. About 3 m to the west of the largest slump fold is another smaller slump about 0.5 m thick by 1.2 m wide. This fold is also has complex internal deformation and recumbant, multiply folded laminations. The slumps appear to be part of a formerly continuous coarse sandstone wedge that tapers toward the west, away from the basin margin.

Beds overlying the slumped horizon are thin bedded and laminated gray mudstone and normal-graded sandstone-siltstone couplets. In the upper few centimeters of the mudstone beds a complexly deformed 2-4 cm medium sand bed forms a "train" of recumbent and overturned folds [Fig.xx]. We interpret that the folded sand layer is a result of bed-shearing forces during the mass flow emplacement of the overlying thick sand wedge that forms the base of Unit IV.

Unit III is the product of mass-flow, turbidite-dominated sedimentation in a pro-delta environment. The normal-graded couplets and mass-flow slumps are intebedded with gray mudstone as opposed to finely laminated fossiliferous black shale, indicating the progradation of the fan-delta over organic-rich lake bottom sediment.

    Unit IV
Unit IV is similar to the underlying Unit III but overall coarser-grained and less dominated by the large slump folds that characterize Unit III. The base of Unit IV is formed by a massive coarse to granule sandstone bed with a wedge-like shape that progressively thins from 40 cm in the eastern portion of the outcrop to 20 cm in the western portion. The lower surface of the sand bed is generally sharp and planar, although some soft sediment load structures are observed. The upper surface is hummocky to irregular and is "onlapped" by normal-graded sandstone beds in the western portion of the outcrop. Internally, the sand bed is massive to laminated; cross stratification is not observed. The normal-graded sandstone beds that overlie the massive sandstone wedge are about 20 to 30 cm thick and consist, internally, of thin, 10 to 3 cm, normal-graded couplets. The apparent "onlap" of these beds on the massive sandstone bed is caused by the westward thickening of the normal-graded beds.

Unit V
Unit V is an abrupt grain-size transition from the underlying turbidite sandstone. Very coarse sand and fine gravel predominate the lower part of the unit and pebbles and cobbles appear in higher abundance toward the top of the unit. A few scattered pebbles and cobbles “float” within the matrix throughout the unit. Typical fluvial sedimentary structures are missing from Unit V, including crossbedding and lag gravels, nor are typical deltaic features, such as, climbing ripple cross-laminae, load casts, or sorted layers and lenses, present. Furthermore, features of typical sub-aerial alluvial fans, including fining-up layers and lenses with silt drapes, or debris-flow lenses are not in evidence. Therefore the depositional environment of the Unit remains somewhat enigmatic, although a sub-lacustrine origin seems likely.

Unit VI
Unit VI is a spectacular, coarsening-up cobble and boulder conglomerate. The conglomerate may be observed in three dimensions, including breathtaking bedding plane views on the south side of the outcrop. The largest clasts, ranging up to 2 meters in length, are observed in the highest stratigraphic levels in the outcrop. The polymict conglomerate includes both low- and high-grade metamorphic rocks derived from the Paleozoic eastern highlands; phyllite, gneiss, and quartzite are all common, and one basalt clast was observed. Stratification within Unit VI is obscure. In many places grain size segregations suggestive of bedding are observed, but, except in a few locations, crossbedding is generally faint or absent. As in Unit V evidence of sub-aerial alluvial fan deposition, including, but not limited to, well-defined channels, silt drapes, or debris flow lobes, is absent. This unit was likely deposited at and below the lake margin where rapid sedimentation below water resulted in poorly sorted and poorly segregated, amalgamated conglomerate with faint bedding.

Rift basin context
The Triassic-Jurassic Hartford Basin is an elongate half-graben with a fault-bounded eastern margin and is one of a series of related rift basins found in onshore and offshore areas along the eastern flank of the Appalachian orogen from Florida to the Canadian Maritime provinces. The basin stratigraphy is roughly tripartite with a lower coarse fluvial sequence, the New Haven Formation; a middle fluvial and lacustrine sequence consisting of the Shuttle Meadow, East Berlin and lower Portland formations; and an upper coarse fluvial sequence, the upper Portland Formation. Three, thick basalt flows extruded in the early Jurassic now form prominent stratigraphic markers delineating the sedimentary formations. The Triassic-Jurassic boundary occurs below the Talcott Basalt and within the uppermost portion of the New Haven Formation fluvial sequence.
[Fig HB + strat]

In studies of the Jurassic strata of the Hartford basin LeTourneau (1985), LeTourneau and McDonald, (1985); McDonald and LeTourneau (1988, 1990), using paleocurrents, sediment provenance, facies analysis, stratal geometry, modeled configuration of the hinged and faulted basin margins, and basalt flow directions (Ellefsen and Rydel, 1985), proposed an asymmetrical half-graben basin model. The syn-depositional structural configuration of the Hartford Basin exerted a strong control on the distribution of depositional environments mainly by the creation of tectonic slopes. Asymmetrical subsidence of the basin placed the deepest portions of the depositional basin adjacent to the fault-bounded eastern margin (e.g. moat graben of Leeder, 1988). Hettangian-age fossiliferous black shale units invariably thicken toward the basin margin where they intercalate with coarse littoral and alluvial fan deposits. (Fig from thesis). Footwall uplift resulted in short, steep drainages which shed sediment to small basin margin alluvial fans (FIG drainage model). The presence of a few larger alluvial fan complexes may be a result of the capture of established antecedent drainages, or their location at breached footwall drainages at overlapping segments of basin margin normal faults (Johnson, Wells and Scholtz, 1995; Smoot, 1995; Anders and Schlishe, 1996). On the western, hanging wall, or hinged margin of the basin, larger watersheds contributed substantial sediment to the subsiding basin. Sediment derived from axial regions is also a major source of basin fill. Paleogeographic analysis indicates that broad areas of the western portion of the basin sloped east toward the off-axis depocenter. An idealized lacustrine interval has shallow water deltaic and nearshore carbonate-rich deposits on the hinged margin and thick, deep-water dark shale intercalated with littoral sand and alluvial fan conglomerate on the footwall side (McDonald and LeTourneau, 1988; Scholtz and Rosendahl, 1990) . This stop illustrates the relationship of footwall-sourced fans with local drainages and deep-water lacustrine strata.

This paleogeographic model for the Hartford rift basin is supported by studies of modern rift basins in East Africa and structural models based on modern and ancient rifts which suggest that extended terranes share a similar tectonic framework (Rosendahl et al., 1986; Rosendahl, 1987; Leeder, 1988; Kusznir and Egan, 1989; Scholtz and Rosendahl, 1990; Kusznir et al. 1995; Schlische and Anders, 1996;). Asymmetric half graben are the most common form of rift basin (Rosendahl, 1987). Due to differential isostatic loading, the footwall of the master border fault system undergoes profound uplift during extension, resulting in small, steep catchments on the faulted margin (Jackson and McKensie,1983; Leeder et al., 1988; Sholtz and Rosendahl, 1990). The basin depocenter is skewed toward the faulted margin and the broad area of the hinged or platform margin containing major inflowing streams slopes toward the faulted margin. Most basin-filling sediment is derived from hanging wall and axial sediment sources; footwall sources are only of local importance. Pre-rift drainages may be captured or breach the footwall (Johnson, Wells and Scholtz, 1995; Smoot 1995).

[Fig. model cross section of typical rift basin]

[ Fig. typical high stand facies distribution]
Needs to be reduced

The Early Jurassic Portland Formation in central Connecticut and south-central Massachusetts consists of continental rift basin fluvial, lacustrine and playa lake strata. The lower portion of the Portland strata, found within 1200 vertical meters of the underlying Hampden Basalt, contain interbedded fluvial and lacustrine cycles (LeTourneau, 1985b). Facies analysis of the lacustrine strata reveals a heirarchical arrangement of shallow to deep water sedimentation related to lake trangression, highstand, and regression. The sedimentary structures, grain size distribution patterns, paleontology, clast composition, and facies associations are markedly different in basin margin and central basin areas. Deposits adjacent to the eastern margin are dominated by coarse alluvial and fluvial facies with west-flowing paleocurrents, and thick dark shale beds. Central and western basin areas are dominated by fine to coarse sandstone, siltstone and shale with generally east-flowing paleocurrents. Here, dark shale beds are thin or absent and carbonate-rich shallow lacustrine or playa deposits are more common than in eastern portions.

    Alluvial fans
Well-developed alluvial fan deposits are found along the eastern fault margin of the Hartford Basin from North Branford to South Glastonbury, Connecticut. In particular, the alluvial fan deposits of the lower Portland Formation in Durham, Middletown, and Portland have been extensively studied (Gilchrist 1979, Let 1985a,1985b). These deposits consist of coarse sandstone and conglomerate in coarsening- and fining-up sequences with intercalated, fossiliferous, finely laminated lacustrine dark shale. The alluvial fan deposits form discrete prisms of coarse-grained rocks within finer-grained fluvial sandstone and siltstone, and dark shale lacustrine mudstone deposits of the basin floor. The alluvial fan deposits can be separated into two general types of facies associations. The relatively large (1 to 4 km radius) basin margin alluvial fans, typically show evidence of fluvial activity, such as cross stratification, channels, and thin fining-up beds and features of wave reworking and shoreline modification of fan sediment is often observed.. Large boulders may be found, but these are generally a small percentage of the total conglomerate clast population and they occur in discrete boulder layers, including debris flows, or as isolated clasts. In contrast, the smaller radius fans (0.5 - 1 km), perhaps more related to steep talus fans, are: characterized by extremely poor stratification and internal fabric organization; are exceeedingly coarse-grained, and are thick bedded with little cross-stratification or channelization evident. Large boulders comprise a significant percentage of the conglomerate clasts often within a fine sand and silt matrix.

The contrast in the two fan types indicate differences in the dominant depositional processes (e.g., stream-flow, debris-flow, colluvial) which are linked to source area drainage basin size, catchment and fan slopes, and stage of fan development (Blair and McPherson, 1994, Fig 20, pg. 474). For reference, the alluvial fan deposits of the Portland Formation in central Connecticut have been identified by their assocation with modern geographic features, e.g. Round Hill Fan, Reservoir Brook Fan (LeTourneau, 1985b). The alluvial fan - lacustrine sequence described here is located within the Hales Brook Fan complex.

    Perennial lakes
The intercalated dark shale beds are typically very finely laminated, lack evidence of bioturbation or sub-aerial exposure, contain fine carbonate (calcite and dolomite) laminae, often contain exquisitely preserved fish fossils, and are laterally continuous over broad areas of the Hartford Basin. The dark shale beds are the result of sedimentation in deep-water, stratified, perennial lakes (Olsen, 1984) Stratification of the water column promotes anoxic bottom water conditions which in turn allows the preservation of abundant organic matter and excludes benthic epifauna and infauna resulting in finely laminated, organic-rich black shale with fully articulated fish fossils. As demonstrated in the Newark Basin and the East Berlin Formation in the Hartford Basin, the periodic occurrence of perennial lakes within the vertical stratigraphic section of these basins is a result of astronomical forcing (Milankovitch cycles) of regional climate in the late Triassic and early Jurassic (Olsen, 1984, 1986, 1996; Olsen et al. 1989). Therefore, the presence of fossiliferous dark shale beds within conglomerate beds represent the climatically controlled transgression, high stand, and regression of perennial lakes over coarse basin margin alluvial fan deposits.

    Fan Deltas
Fan delta deposits are formed where alluvial fans intersect marine or perennial lake waters (Wescott and Ethridge, 1990). Distinct from wholly sub-aerial alluvial fans, fan deltas consist of two portions: 1) the sub-aerial alluvial fan deposited above mean water level and: 2) the sub-aqueous portion deposits below standing water (Nemec and Steel, 1988; Blair and McPherson, 1994) . Although alluvial fans are deposited by water-laden mass flows, ephemeral to perennial stream flow, and sheetfloods, in this context "sub-aerial" means all deposition above mean water level of a perennial lake. The sub-aqueous portions of fan deltas show evidence of coarse-grained deposition into profundal waters dominated by fine grained sedimentation, and typically contain turbidite beds and soft sediment slumps and folds generated on unstable sub-aqueous slopes (Postma, 1984). Fan delta sequences typically coarsen-upward as a result of the progradation of coarse-grained littoral, shoreline, and alluvial fan sediment. These features are well represented in the Hales Brook Fan fan delta sequence. A progradational sequence may result from sediment deposition on the alluvial fan during relatively static lake levels or from the "forced" regression of lake margin deposits during receding lake levels. I interpret the Hales Brook Fan delta as the progradation of an alluvial fan into deep water during a lake high stand because of the intimate association of exceedingly coarse deposits with anoxic, profundal lake water and the absence of features indicative of sub-aerial exposure. Other alluvial fan deposits in the lower Portland Formation show evidence of progradation following climate-driven lake regression, but description of these is beyond the scope of this paper.