Question: What event in 1170 may have caused the creation of 5,000 km2 large shallow bay of the North Sea in the northwest of the Netherlands?

Context articles: All Saints' Flood (1170)
North Sea Wind Power Hub
Turgai Sea
Paratethys
Gulf of Mannar
Breiðafjörður
Gold Coast Broadwater
Geology of southern North Sea
Zuiderzee
Southern Bight

The All Saints' Flood of 1170 (Allerheiligenvloed) was a catastrophic flood in the Netherlands that took place in 1170.
 Large parts of the Northern Netherlands, and Holland territories were overflowed.
 This is the flood that created the Zuiderzee.
 "Lake Flevo" was once a fresh water lake, but after this flood a sea channel opened a connection from the North Sea into the lake through Creil Woods, and turned the lake into the salt-water sea known as the Zuiderzee.
 The Creiler Forest vanished forever under the waves.
North Sea Wind Power Hub is a proposed energy island complex to be built in the middle of the North Sea as part of a European system for sustainable electricity.
 One or more “Power Link” artificial islands will be created at the northeast end of the Dogger Bank, a relatively shallow area in the North Sea, just outside the continental shelf of the United Kingdom and near the point where the borders between the territorial waters of Netherlands, Germany, and Denmark come together.
 Dutch, German, and Danish electrical grid operators are cooperating in this project to help develop a cluster of offshore wind parks with a capacity of several gigawatts, with interconnections to the North Sea countries.
 Undersea cables will make international trade in electricity possible.
The Turgai (or Turgay) Sea or Turgai Strait, also known as the West Siberian Sea, was a large shallow body of salt water (an epicontinental or epeiric sea) of the Mesozoic and Cenozoic Eras.
 It extended north of the present-day Caspian Sea to the "paleo-Arctic" region, and was in existence from Middle Jurassic to Oligocene times, from approximately 160 to 29 million years ago.
The Paratethys ocean, Paratethys sea or just Paratethys was a large shallow sea that stretched from the region north of the Alps over Central Europe to the Aral Sea in Central Asia.
 The sea was formed during the Oxfordian stage of the Late Jurassic as an extension of the rift that formed the Central Atlantic Ocean and was isolated during the Oligocene epoch (about 34 million years ago).
 It was separated from the Tethys Ocean to the south by the formation of the Alps, Carpathians, Dinarides, Taurus and Elburz mountains.
 During its long existence the Paratethys was at times reconnected with the Tethys or its successors, the Mediterranean Sea or Indian Ocean.
 From the Pliocene epoch onward (after 5 million years ago), the Paratethys became progressively shallower.
 Today's Black Sea, Caspian Sea, Aral Sea, Lake Urmia, Namak Lake and others are remnants of the Paratethys Sea.
The Gulf of Mannar is a large shallow bay forming part of the Laccadive Sea in the Indian Ocean.
 It lies between the southeastern tip of India and the west coast of Sri Lanka, in the Coromandel Coast region.
Breiðafjörður (lit.
 "widening fjord") is a large shallow bay, about 50 km wide and 125 km long, in the west of Iceland.
 It separates the region of the Westfjords (Vestfirðir) from the south of the country.
 Breiðafjörður is encircled by mountains, including the glacier Snæfellsjökull on the Snæfellsnes peninsula to the south and the West Fjord peninsula to the north.
 An interesting feature of the bay is that the northern tip was formed about 15 million years ago, whereas the southern end at Snæfellsnes was formed less than half that time ago.
The Gold Coast Broadwater, also known as Southport Broadwater, Gold Coast Harbour and The Broadwater, is a large shallow estuary of water located in the Gold Coast district of South East Queensland, Australia.
 The estuary reaches from the locality of Southport in the south, to the southern section of the UNESCO World Heritage Listed Moreton Bay in the north.
 Separated via the Seaway from the Coral Sea by a thin strip of land called Stradbroke Island, the original body of water was a lagoon created from water deposited from the Nerang River.
 Part of the Broadwater is contained within the Moreton Bay Marine Park.
The North Sea basin is located in NE Europe and lies between the United Kingdom, and Norway just north of The Netherlands and can be divided into many sub-basins.
 The Southern North Sea basin is the largest gas producing basin in the UK continental shelf, with production coming from the lower Permian sandstones which are sealed by the upper Zechstein salt.
 The evolution of the North Sea basin occurred through multiple stages throughout the geologic timeline.
 First the creation of the Sub-Cambrian peneplain, followed by the Caledonian Orogeny in the late Silurian and early Devonian.
 Rift phases occurred in the late Paleozoic and early Mesozoic which allowed the opening of the NE Atlantic.
 Differential uplift occurred in the late Paleogene and Neogene.
 The geology of the Southern North Sea basin has a complex history of basinal subsidence that had occurred in the Paleozoic, Mesozoic, and Cenozoic.
 Uplift events occurred which were then followed by crustal extension which allowed rocks to become folded and faulted late in the Paleozoic.
 Tectonic movements allowed for halokinesis to occur with more uplift in the Mesozoic followed by a major phase of inversion occurred in the Cenozoic affecting many basins in NW Europe.
 The overall saucer-shaped geometry of the southern North Sea Basin indicates that the major faults have not been actively controlling sediment distribution.
The Zuiderzee (] ; old spelling "Zuyderzee") was a shallow bay of the North Sea in the northwest of the Netherlands, extending about 100 km (60 miles) inland and at most 50 km (30 miles) wide, with an overall depth of about 4 to 5 metres (13–16 feet) and a coastline of about 300 km (200 miles).
 It covered 5,000 km2 .
 Its name means "southern sea" in Dutch, indicating that the name originates in Friesland, to the north of the Zuiderzee (cf. North Sea).
 In the 20th century the majority of the Zuiderzee was closed off from the North Sea by the construction of the Afsluitdijk, leaving the mouth of the inlet to become part of the Wadden Sea.
 The salt water inlet changed into a fresh water lake now called the IJsselmeer (IJssel Lake) after the river that drains into it, and by means of drainage and polders, an area of some 1500 km2 was reclaimed as land.
 This land eventually became the province of Flevoland, with a population of nearly 400,000 (2011).
Southern Bight is the southern bight of the North Sea bounded by the coasts of the Netherlands, Belgium, France and Great Britain.
 The Southern Bight is south west of the German Bight and the Wadden Sea.
 The Southern Bight is roughly delimited in the north by the line The Wash-West Frisian Islands, south of the Dogger Bank, a large shallow part in the North Sea, and the Outer Silver Pit, a deep water channel south of the Dogger Bank.