4. Marine Deposits

Types of marine deposits and their distribution

Carbonate sediments deposited by the sea and preserved as ancient marine limestone are scarce on Bermuda relative to wind blown dunes, or eolianites. The reason for this is that sea levels over the last two million years or so were predominantly much lower than today. Most marine deposits are therefore submerged.

The majority of marine limestones that do exist on Bermuda are concentrated in a narrow zone along the south shore in the central parishes. Although they can be similar in appearance to eolianites, they may be distinguishable on the basis of their stratification or the size of their particles, or clasts. Pebbles, cobbles or large shells, for example, can be readily transported by wave action in the sea, but not by the wind (Figures 4a and 4b).

Long Island Conglomerate
Figure 4a. A marine conglomerate on Long Island in the Great Sound. This accumulation of limestone cobbles and rounded boulders ranging in size up to 0.3m (1 ft) could only have been deposited by the sea at a location that was exposed to wave action. The age of this marine conglomerate – approximately 120,000 years – has been determined by chemical analyses of coral fragments found within.  Situated at an elevation of up to 2 m above present sea level, this deposits would have accumulated during the last inter-glacial period  when sea level was relatively high.
Coquina (2).jpg
Figure 4b. Fossil sea shells on Agar’s Island in the Great Sound. This concentrated accumulation of shells, or coquina,  was deposited in the sea at a time, approximately 120,000 years ago, when sea level exceeded that of today. Because the shells  have  been transported by currents  since their death, and  are clearly not preserved in the position in which they lived, this accumulation is known as a “death assemblage”.

Deposits of ancient marine limestone on Bermuda, and the environment in which they accumulated, can be identified through comparison with modern sediments. “The present is the key to the past”  is the principle on which this approach is based.

The three main types of ancient marine deposits on Bermuda are:

  1. Beach (foreshore) deposits. Many of Bermuda’s marine deposits accumulated on the foreshore as sandy beaches, and are characterised by long parallel strata which dip down at approximately 10° towards the sea (Figures 4c and 4f). These strata represent the “beach face” or “swash zone”, which can be observed on modern beaches as a seaward sloping wave-washed ramp. At the top of the beach face is the “berm” – a near horizontal platform which transitions landward into the dry upper beach (popular with sunbathers).  The landward end of the upper beach merges into vegetated dunes, known as foredunes (Figure 3a, Chapter 3) . In the geological record, the berm and upper beach are represented by near-horizontal strata which can feature trace fossils of crab burrows (Chapter 9) .
  2. Sub-tidal (shoreface) deposits. These deposits form below sea level immediately offshore from the beach in a high energy zone subject to wave action and associated oscillating currents . The rippled sediment surface that forms here is recorded in the geological record by a variety of small to medium scale cross-strata which dips at up to ~20° to horizontal in a variety of directions (Figure 4d).
  3. Erosional debris. Rocky shores where beaches are not forming are subject to wave erosion. Much of the product of this erosion is transported away, along the shore or into deep water. Occasionally it is preserved as a plaster on the surface, or in a notch, of a rocky shore. This accumulation of eroded debris consists primarily of limestone fragments ranging in size from pebbles to boulders. Held together in a matrix of cemented sand, they are preserved in the geological record as a “conglomerate” or “breccia”,  (Figures 4a, 4e) .
Belmont Beach annotated.jpg
Figure 4c. Ancient beach deposits on the South Shore, east of  Hungry Bay. These marine deposits of an ancient beach  are recognisable by the near-parallel seaward dipping strata of the beach face (A) and the flat crab-burrowed deposits of the beach berm (B). At the time of deposition approximately 200,000 years ago, sea level was slightly higher than today and rising. These beach deposits of the Belmont Formation extend along the south shore from Hungry Bay to Watch Hill Park.
Spittal pond ripples
Figure 4d. Sub-tidal marine deposits at  Spittal Pond West,  Smith’s Parish. Belmont Formation. The “trough cross-stratification” in the lower half of this 1 m (3 ft) high rock exposure represents oscillation ripples formed by waves approximately  1 m or more below average sea level. This transitions upwards into seaward (southward) dipping strata most likely representing the beach step which would have formed just below average sea level.
Unconformity
Figure 4e. A marine conglomerate at Grape Bay, Paget Parish.  An accumulation of limestone fragments, shells and sand form a  marine conglomerate, which overlies the irregular eroded upper surface of a much older, well cemented limestone. The latter is the Belmont Formation and the former is the Devonshire marine  member of the Rocky Bay Formation. Ages determined from coral fragments are respectively ~200,000 years and ~120,000 years. The conglomerate accumulated as sea level rose at the beginning of the last Inter-glacial period. It transitions upward into a sandy limestone identified by its strata (not clear here) as a sub-tidal deposit. These Rocky Bay sediments, thus, record deepening water (rising sea level) approximately 5 m above present sea level. (Scale: 0.15 m or 6 in rule provides scale)

The significance of Bermuda’s ancient marine deposits

The significance of ancient marine limestones preserved “high and dry” on Bermuda today, is that they record positions of relative* sea level which were as high or higher than that at present. By establishing the age of these  marine deposits, through dating of coral fragments (Chapter 9), valuable knowledge of  sea level history over the last half million years has been acquired. Pleistocene sea-level data from Bermuda has, because of its assumed global significance, featured in numerous articles in highly respected scientific journals and has been the subject of much debate* (LA2, HA1,VA2,HE2, HE5,MU2,RO4).

*The debate has increasingly focused on the degree to which the elevation of palaeo-sea-level imprints on Bermuda have global significance. Repeated assertions that the Bermuda seamount has acted as a stable dipstick or tide-gauge are now considered questionable, at best. The seamount itself has been subjected to tectonic (Chapter 10) and isostatic (Chapter 8) forces which have caused vertical movements of the islands throughout their history, including the Pleistocene Epoch. This disqualifies Bermuda as a datum against which changes in eustatic global sea level can be precisely measured (< ± 5 m). All sea level data from Bermuda including those derived from marine deposits, quoted herein and elsewhere, should be considered “relative” (to a datum on Bermuda).

Beach and sub-tidal deposits as (relative) sea level indicators

The form, or topography,  of a sediment surface – be it rippled, flat or undulating – is diagnostic of the environment in which the sediment was deposited. In ancient marine deposits, the  geometry of the stratification exposed in a rock face, represents the changing topography  of the sediment-surface. The stratification, therefore, informs us about characteristics of  the depositional environment, such as the energy of waves or currents and the water depth, or relative sea level (Figure 4d).

The beach face is recognisable in the geological record by its planar strata which slope seaward at an angle of 10 degrees or less. (Figure 4c). Beach face strata cannot, however, be conclusively identified unless they are found in physical contact with other deposits which are members of the beach system, such as the upper beach, or berm, or sub-tidal deposits of the upper shoreface (Figure 4f). Depending on the wave conditions at the time of deposition, the top of the beach-face in a microtidal environment, such as   Bermuda, can vary in height from less than 1 m (3 ft) to greater than 2 m (7 ft) above mean sea level. The elevation of the lowest strata of the beach face is a more stable indicator of sea level. On Bermuda’s modern beaches, the base of the beach face is at approximately 0.5 m (2 ft) below average sea level.

Spittal Pond Beach Face
Figure 4f. An ancient beach system at Spittal Pond West, Smith’s Parish.  These  Belmont Formation shoreline deposits are viewed, here, from the seaward side. They  comprise:  sub-tidal beach step of the upper shoreface (Bs);  beach face (Bb);  fossil soil or protosol (Bp) and dune (Bd). From our knowledge of modern beach morphology, the position of contemporaneous average sea level can be inferred. It would have been just above the top of the sub-tidal deposits (Bs), at the base of the beach face (Bb) or just over 3m (10 ft) above present sea level. (Scale: Dunes (Bd) are approximately 5m (15 ft) high).

To seaward of the beach in water depths of less than two metres is a high energy environmental zone known as the upper shoreface. Sub-tidal sand ripples formed by waves or tidal  currents in this zone are recorded in the geological record by chaotic, truncated sets of small scale cross-strata (Figure 4d).

The beach step is a sub-tidal feature of the upper shoreface. On many modern beaches, it comprises a short steep ramp which is  created by the scouring action of breaking waves at the base of the beach face (Figure 4g). In the geological record it is represented by steep parallel, seaward dipping strata of less than 1 metre length usually found below beach face strata (Figure 4f). Because formation of a beach  step is constrained vertically within a narrow range close to average sea level, the beach step when preserved in the geological record is a robust indicator of past sea level elevation (DA2,RO4) (Figure 4h).

Beach step vortex.jpg
Figure 4g. Formation of a beach step at Western Warwick Long Bay.  A backwash vortex is created at the base of the beach face, where water returning to the sea (flowing from left to right)  meets a small oncoming wave (approaching from right to left). As a result of the scouring action created by the vortex,  a short steep ramp or “step” at the base of the beach face is constructed. The change in depth from the left side of the vortex  to the right side is evident when viewed through clear tropical waters, as here. The unique characteristics of the beach step and close spatial association with average sea level make it an important marker of past sea-level positions, when it can be identified in the geological record.
Grape Bay beach step annotated
Figure 4h. Ancient beach step preserved at Eastern Grape Bay.  Ancient marine   deposits at this locality comprise a vertical succession (upward) as follows: wave ripples (A) of the upper shoreface, steep seaward dipping strata of the beach step (B) and low angle planar strata of the beach face, or foreshore (C). The elevation of the the top of the beach-step strata of 3.3 m (10ft) at this locality attests to an average sea level elevation at the time of deposition of approximately 3.5 m  above present sea level. This would have been approximately 200,000 years ago based on the age of coral fragments collected from the deposit. (Scale: 15 cm (6″) brown rule standing on its end  provides scale)

Conglomerates and erosional features as sea level indicators

Cemented conglomerates which comprise wave-worn erosional debris are found at numerous localities along Bermuda’s south shore in Paget, Devonshire and Smith’s Parishes. Through U-series dating of coral fragments found therein, the majority of these conglomerates have been assigned to the Rocky Bay Formation (Age: ~120,000 years) and a few to the Belmont Formation (Age: ~200,000 years). Older conglomerates do exist (HA1,HE2) but the age data is nowhere near as reliable as for the younger deposits.

Marine conglomerates, preserved five to ten metres above present sea level on Bermuda’s shores,  have been enthusiastically embraced as sea level markers – more so than is perhaps justified. Only when found in combination with some other subtidal, intertidal or supratidal feature – can the elevation of a conglomerate be considered of value to high resolution sea level studies – i.e. to an accuracy of one or two metres. A distinctive erosional “notch”, or “shoreline angle”, is potentially such a feature (Figure 4i). If correctly identified, it is (Figure 4j) is diagnostic of past marine erosion at an elevation which is within reach of seasonal storm waves i.e. 1 to 2 metres above mean sea level.

Doe Bay notch.jpg
Figure 4i. A modern erosional feature near Doe Bay, South Shore Devonshire Parish. This  notch, partially infilled with wave-worn erosional debris, is situated approximately 1.5 metre (5 ft) above present sea level in the supra-tidal zone. The characteristics of the feature are consistent with frequent assaults by storm waves.  (Scale: yellow 1 m rule provides scale)
Watch Hill Park Conglomerate
Figure 4j. An ancient erosional feature at Watch Hill Park, Smith’s Parish.  This notch, infilled with cemented erosional debris, is situated at 6m above present sea level. It shares key characteristics with the modern example shown in Figure 4i. Since it is cut into limestone of the Belmont Formation, it is evident  that a later sea level – probably of the Last Interglacial  ~120,000 years ago – reached 5 m (16 ft) or more above present mean sea level.

The most conclusive combination of a conglomerate and an erosional notch on Bermuda occurs on the east-facing coast of St. David’s Island. Cut into a Town Hill Formation cliff, the flat-topped erosional notch is infilled with a pebbly conglomerate at approximately 5 m above present mean sea-level (Figure 4k). Based on the planar nature of its upper horizontal surface, it can be concluded that the notch does not represent storm-wave erosion but, rather, protracted inter-tidal mechanical and biological erosion associated with a lingering sea level “highstand”. There is no age data for the feature, but because of its good state of preservation on a very exposed stretch of shoreline, it is assumed to be young and therefore, likely, a product of the Last Interglacial period. The infill material would therefore be assigned to the Rocky Bay Formation marine member.

St David's Notch
Figure 4k. Marine notch on the east coast of St David’s Island.  Two Town Hill Formation dune units (A and B) are exposed at this locality. The younger one (B) exhibits an infilled erosional notch (C). It was most likely incised by a combination of wave and biological erosion at a sea level highstand thought to date to the Last Interglacial period approximately 120,000 years before present. The notch situated at approximately 5 m above present sea level is infilled with a pebbly conglomerate. a caption