Solnhofen and the inter-tidal mudflat at Ha Pak Nai, Hong Kong – Environmental factors

This post is one in a series of posts which compares the depositional environment of the late Jurassic lithographic limestones at Solnhofen in Germany based on extracts from Solnhofen: A Study in Mesozoic Palaeontology with observations made at a modern inter-tidal mudflat at Ha Pak Nai, Deep Bay, New Territories, Hong Kong and proposes an inter-tidal mudflat origin for the examples cited.

This post gives an overview of some of the factors influencing life on the inter-tidal mudflat at Ha Pak Nai, Deep Bay, West New Territories, Hong Kong.

Geology

Ha Pak Nai lies at the south end of Deep Bay in the North West New Territories in Hong Kong. The underlying rocks in this area are fine to medium grained Jurassic granite, giving way to superficial geological deposits along the coast including silts, sands and gravels, which form pockets of low lying coastal plain along the edge of Deep Bay.

The land backing Ha Pak Nai comprises significant areas of eroded natural hillside, where chemical weathering has not only resulted in large scars on the hillsides, but also serves to constantly feed the coast with new sediment in the form of quartz grains, minerals and clay materials, washed down by the streams which run off these granite hills (Langford et al 1989).

The coastal habitats around the stream mouth regularly change in nature due to a variety of influences.

Seasonal variations

The coastal area of Ha Pak Nai is fluvial dominated delta in summer, dominated by beach and deltaic channel facies. In winter, tides and waves rework and sort the sediment, with seasonal lagoons, sand bars, sandy flats and mudflats developing. Beach deposits have formed in coastal embayments (Lee 1999).

Monthly variations

Each month Ha Pak Nai is subject to Spring Tides (semi-diurnal tide of large range occurring when the moon is new or full) and Neap Tides (tides of small range when the moon is at its first or last quarter). The tides in Hong Kong range from extreme lows of below 0.00 metres to highs of 3.1 metres, although in Ha Pak Nai they usually range between 0.1 to 2.7 metres.

At certain times of the year, the tide cycles result in abnormally low high tides, which leave large areas of the mudflat exposed for a number of days. This phenomenon may be a factor in some of the fossil preservation at Solnhofen.

Daily variations

Ha Pak Nai is subject to semi-diurnal tides – a tidal pattern in which there are two high tides and two low tides each day. Mudflat animal behavior is synchronized with these tidal cycles. As the tide starts to come in, animals retreat to their burrows or bury themselves. After the tide recedes, and the mudflat is exposed, they emerge to forage.

Photographs of Ha Pak Nai 

Aerial view of Ha Pak Nai showing eroded granite hills feeding the coastal area (Image: Google Earth)

Aerial photograph of steam mouth at Ha Pak Nai at low tide, showing position of seagrass beds in relation to stream mouth. (Image: Aerial Photograph. Ha Pak Nai. 2 December 2008. Survey and Mapping Office, Lands Department, HKSAR Government)

Stream mouth at Ha Pak Nai at low tide.

Sandy Beach (A) at Ha Pak Nai. 

Sand bars at stream mouth at Ha Pak Nai at low tide.

Seagreass Beds A showing pools forming at low tide between the mounds.

Seagrass Beds B showing pools forming at low tide between the mounds.

References

Langford, R.L., Lai, K.W., Arthurton, R.S. & Shaw, R. (1989). Geology of the Western New Territories. Geotechnical Control Office, Hong Kong. (Hong Kong Geological Survey Memoir No 3).

LEE, M.F. (1999). Sedimentary facies of fluvial-marine transition environments in Hong Kong: Ting Kok and Pak Nai deltas. MPhil Thesis. University of Hong Kong.