According to the GeoParquet specification, the default CRS can be null and should default to OGC:CRS84.
PROJJSON object representing the Coordinate Reference System (CRS) of the geometry. If the field is not provided, the default CRS is OGC:CRS84, which means the data in this column must be stored in longitude, latitude based on the WGS84 datum.
According to the GeoParquet specification, the default CRS can be null and should default to OGC:CRS84.
https://github.com/opengeospatial/geoparquet/blob/main/format-specs/geoparquet.md#column-metadata