Beach microcosms as fan delta analogues

Overlapping micro fan deltas abandoned by the tide. Field of view about 1.5 m.

Natural analogues for full-sized fan deltas

Walk any sandy beach at low water, negotiating flotsam, cusps, bedforms, and pools draining remnants of the previous tide. I can spend hours wandering these microcosmic forces of nature, to the chagrin of anyone accompanying me (including the dog). Here we are witness to grand geological processes, in miniature – bedform migration, flow regimes and stream channelling, incision, and avulsion, and delta evolution in concert with changing baselevel.

In some respects, these microcosms are the natural cousins of sand-box analogue experiments (the lab type) – but without the controls. Experimental analogues generally have well-defined boundary conditions (physical, mathematical, and conceptual), are scaled appropriately, and are capable of controlling variables such as sediment flux, baselevel, and in the more sophisticated models, subsidence. Such experiments are designed to quantify processes that govern much larger-scale depositional systems. In comparison, beach analogues are purely qualitative and, at least for some of us, non-quantitatively enjoyable.

Take a look at some of the great tank experiments conducted by the stratigraphy – sedimentary basins group at St. Anthony Falls Laboratory (Univ Minnesota).

The scenarios developed here all formed in tidal pools during baselevel fall, lowstand, and dwindling sediment supply on an outgoing tide. The pools are gradually partly or completely drained as the tide recedes. Base level rise during the next flood tide obliterates most of the structures, redistributing the sand to bedforms such as ripples and larger sandwaves.

The sand is fine- to medium-grained, extremely well sorted, and composed of feldspar, magnetite, ilmenite, and various ferromagnesians (typical black, iron-sand beaches along some west coast New Zealand).

The river system

Overland flow on many beaches commonly begins where subsurface seepage exits about midway down the exposed beach face. Channel systems are usually braided. Channels migrate across the braidplain during baselevel fall, controlled primarily by subtle changes in local gradient (an autogenic process), and the competition between surface flow and seepage into the underlying sands.

Channels incision keeps pace with baselevel fall. The depth of incision increases downstream. Close to the delta, these incisions tend to be confined to a relatively narrow part of the braidplain where successive episodes of downcutting leave a flight of abandoned river terraces.

Typical braided channel patterns reflect active channel migration and avulsion, mid-channel bars and chute cutoffs. The latest channels (top if the braidplain) cut across earlier channel reaches. Field of view 1.2 m. — Typical braided channel patterns reflect active channel migration and avulsion, mid-channel bars and chute cutoffs. The latest channels (top of the braidplain) cut across earlier channel reaches. Field of view 1.2 m. Flow is towards the top.

The fan delta

Fluvial channel formation and incision produces sediment that is deposited concomitantly as a micro-fan delta in the adjacent standing body of water (tidal pool). Delta progradation occurs mainly during falling baselevel. The delta top is essentially the downstream extension of the braidplain.

Fine-grained sand is carried by the channels to the shoreline-foreslope break, whereupon it tumbles down the delta front, or foreslope. Under steady-state conditions, the foreslope accumulates at an angle of repose consistent with water saturated fine-grained sand. However, if flow rates and sediment flux vary, the upper slope may oversteepen and collapse, creating small slides and grain flows. As baselevel continues to fall, the shoreline-foreslope break may become incised, forming shelf-edge like gullies that provide focal points for continued offshore sediment supply.

The distal end of the fluvial braidplain merges with multiple fan delta lobes. The flight of terraces developed as channels became incised during baselevel fall – 1 is the oldest. The adjustment of the fluvial channels to falling baselevel has extended far upstream. The most recent, active delta lobe (4) is also the lowest topographically, although it too is undergoing the early stages of incision as the rate of ‘sea level’ fall outpaces sediment supply.

The system of braided channels extends to the fan delta shoreline. Channel incision at the shoreline – delta foreslope break has created gullies that focus sediment transfer offshore. Note the small rotational failures on the steep left bank of the braidplain (top of photo). There are three delta top – distal braidplain terraces – 1 the oldest, and 3 the active system.

Stream flow volumes gradually decrease over the lifespan of these systems, which means that sediment supply also dwindles. The slow-down depends on the up-dip availability of water in the saturated zone beneath the beach surface (the primary source for stream flow); the availability decreases as the beach sands are drained. However, the supply of water may be augmented by local fresh groundwater. In this situation sediment supply remains sufficiently high during the latter stages of baselevel fall such that the delta continues to prograde, producing a downward-stepping succession that is reminiscent of forced regressive wedges.

Two, merged fan delta lobes, the active one at top centre. In this scenario, delta progradation has kept pace with sediment supply (or close to it) during baselevel fall, resulting in a narrow zone where delta growth extends beyond an earlier shoreline. The new shoreline is located at the active shoreline-foreslope break. This pattern of downstepping progradational shoreline packages is analogous to a forced regressive systems tract. The presently active channel continues to supply sand, forcing the shoreline downward and basinward onto a second forced regressive package.

At its lowest, baselevel has usually fallen well below the delta top – foreslope margin. The exposed foreslope may also oversteepen and collapse because of wave action. Baselevel rise, during the ensuing flood tide erodes the deltas and redistributes the sand to other bedforms such as ripples and small sandwaves. An entirely new generation of rivers and deltas will form during the next cycle.