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?

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. <SEP> 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. <SEP> 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. <SEP> 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).