U.S. Silica Holdings Inc. filed SEC Form 8-K: Notice of Delisting or Failure to Satisfy a Continued Listing Rule or Standard; Transfer of Listing, Changes in Control of Registrant, Leadership Update, Amendments to Articles of Incorporation or Bylaws; Change in Fiscal Year, Completion of Acquisition or Disposition of Assets, Entry into a Material Definitive Agreement, Creation of a Direct Financial Obligation, Material Modification to Rights of Security Holders